Bicycle is harder to pedal when standing From a few days now, when trying to pedal standing, my pedals become very rigid and it sounds like if the brakes are dragging. If I sit down again, the noise goes away and it's easy to pedal again.
In fact, if I just try to apply more pressure to the pedals to go faster, the problem happens again.

I thought that the brakes were the problem so I adjusted them and I know they are not dragging.
The chain and gears are well lubricated.
I'm trying to fix this by myself so I can learn. Can you help me to figure out what the problem is?
Something is dragging as you pedal harder. The most likely thing is that your rear wheel is moving so that the tire drags against the chain stay when you apply power.

This is something that you'll probably need to troubleshoot when you're riding because it isn't likely to show up with the bike on a work stand. You might be able to make it happen by applying a standing load to the pedals while holding the front brake. To do this I'd want somebody holding the bars to stabilize the bike (and to help me look for what moves when I load the chain).

The interesting thing about what's happening is that you say the problem goes away when you ease up. If the problem is due to the tire rubbing on the chain stay then there must be some mechanism to return the wheel to its neutral location. The only thing that I can think of right now that might do this is a nutted rear axle where the nut on the right (drive) side is loose enough to allow the axle to move under load and then move back when the load eases up.