In my previous life I was --- I can accept the concept of Reincarnation in Buddhism . That is , after a person died  , then he/she will reborn again as human , as animal , or even as God , in the same / different Realm. But there's one thing that to me is weird . In my previous life I was a mosquito ? , a flea ? , a worm ? , a bacteria ? , a bamboo tree?
Technically speaking... (the devil is in the details!)


You can only 'reincarnate' into a sentient being. Mosquitoes, fleas, and worms are, in general, considered sentient beings in Buddhism. Bacteria (not an animal) and bamboo trees (a plant) are not. One might discuss if it's possible to 'reincarnate' as an amoeba (technically an animal — from the perspective of Western biology, at least), but I'd guess that one would require at least a minimal nervous system with a few cells in order to classify as 'sentient'. Then again, I might be wrong, of course!
'Reincarnation' is a Western term coming from a Christian tradition, with a very specific meaning, so it may lead Buddhist practitioners into the mistake of thinking that somehow their minds will be put into the body of a flea or a mosquito; in practice, it's both more complicated and much more simpler than that, and I'll give you the analogy that my own teachers gave me:


Think of a series of candles, all about the same size, the first of one is lit. The light of that candle represents a sentient's being life. Now, eventually, the candle burns out — and you might have to transfer the flame to the next candle using a stick of wood or something. The next candle might outwardly look like the first one, and from the perspective of someone on the room, there is still light around — even if it might come slightly from the left or the right, or be slightly stronger or weaker because even outwards-looking 'identical' candles will have some difference at the molecular level and thus there will be a difference.

You can repeat the process from one candle to the next, and there will always be 'light' while you keep one flame going, but eventually, there will be no more candles to light.

This symbolises how a stream of consciousness works across 'lives'. All the candles give light — but the light is not exactly the same, because each successive candle is slightly different than the previous one. Each candle is its own distinct object — symbolising different bodies if you wish — and there is no shared material/physical components between each candle. Because there is a succession of flames — the previous one becoming the cause for the next one, and so forth we can say that what you do in one life will influence — 'cause' — the next life. But each is started 'from scratch' in the sense that each life is separate and distinct from the previous one, even if it's in very small details (i. e. even two consecutive human lives will be quite different!). There is technically 'nothing' passing from the flame of one candle to another (this is where the analogy fails because it requires the stick/match/a bit of paper etc.), except this very abstract notion of 'light': i.e. all candle flames produce light, and the flame from one candle is the only cause for kindling the flame of another candle, so there is a cause-and-effect tying all candles together (this is the stream-of-consciousness which 'ties' together different lives through a causation effect). And finally, when one reaches awakening/enlightenment, there will be no more candles — no need to 'reincarnate' again, if you wish. So there will always be an end to the cycle of reincarnation; the role of Buddhist teachers is to get as many people to get out of that cycle as fast as possible (through teaching their students the means to do so, of course)!

Is this a confusing analogy? I think so; it is one supposed to make us think a bit. For instance, it's true that you have been a mosquito or a worm on previous lives, but you have no recollection of that — this is simply because not only each candle is physically different, but because its flame is different, so you cannot 'look back' to how you were in the past. Nevertheless, there is a connection: what you did as a mosquito back then has influenced what you are today (in exactly the same way that what you do today will affect you tomorrow — even if you don't notice it in most cases). Of course, a poor mosquito will have little knowledge of how to behave in order to get a 'better' life in the future (what makes you a 'good' or 'bad' mosquito...?); however, you, as a human being, have the ability not only to influence what your next life will be (it only depends on what you do in this life), but, even better than that, you have the abilities and capacity to break free from the reincarnation cycle altogether! When that happens, you can spend the rest of eternity in any form whatsoever and have the ability to watch from afar how flames go from one candle to another... while Buddhas cannot directly influence how that happens (even Buddhas cannot change the causation effects on other beings!), they can certainly help us out with tools and methods to figure out, on our own, how to break free from the cycle of reincarnation.

Patience, young grasshopper, you're on the right path! 

Extra bonus: just by following the Buddha's path will give you another chance as a human being. And you were so fortunate to find the Buddha's path and a teacher on this life because, well, you did great on a previous life! Perhaps, as a mosquito, you might have bitten a Buddha by accident — thus creating a bond with a Buddha, which has ultimately led you to be 'reborn' as a human being able to study the Buddhadharma, think about it, and meditate. So rejoice!