Why does my Rear bike tire slow down when Pedaling? I just replaced a flat on my rear road-bike tire. I had also tried to replace the rear axle, By removing the axle completely, to a quick release lock but realized the axle that had been installed with the bike tire wasn't hollow so put the old axle back on the bike, seemingly correctly. This morning i went for a ride and when I put any pressure while pedaling it felt as if the rear breaks were engaged. I checked to see what would happen if i pedal with the rear tire in the air and the tire seemed fine but came to a stop more quicker than usual but nothing too alarming. 

It seems the problem only occurs when im siting on the bike and there is weight. I also noticed when i pedal the rear derailleur seems to tug forward and hug the rear cog set. im not sure if that's the issue... 

Any ideas? I was wanting to save some money by not going to the LBS and learning and doing it myself. But I may have to go anyways. 

Here are some images of the bike
By far the most likely reason is that you've over tightened the bearings.

Did you feel as if you needed 3 hands to get it back together? If not, you did it wrong. :)

As usual Sheldon has a pretty good writeup on this, but the basics are easy enough.

Put one side back together, tighten it up as best you can. Set the cone on the other side to the correct tension. Tighten locknut on second side without turning either cone.

The tricky part is the last bit. You need to hold the cones on both sides at the same time whilst tightening the locknut and you really need 3 hands.

You will most likely have to try a few times to get the tension just right. When it's done it should turn freely but not have any wiggle.