Your muscles are too weak in that position. I use to get this in the early season this yr when climbing. I'd go push it for some more speed but would bail after feeling the insane lactic acid cramping. The next time I went out, I started off by doing a lot of climbing out of my seat. It is a good idea to cycle about three positions for training I think; normal seated, shift a little back and push with the butt/leg biceps and then of course, standing out of seat.

You just need some time meditating to Rule #5.