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?