Exercising is confusing me

I have a stationary bicycle.  I love my stationary bicycle because it is a very good way for me to exercise due to my bad knees.  What I don't love is when it nearly kills me to ride it when I stop for a couple of weeks.  Literally a couple of weeks.  I literally had to stop 5 times last night while riding it for 25 minutes.  That is just madness, but I couldn't breathe.  Now keeping that in mind, I rode it on Sunday for 30 minutes without a problem in the world.    The only thing I've managed to figure out is that it has something to do with the heaviness of the air?  The more humidity, the more problems I have.  I cycle indoors.  Is this just weird to me? 

I think I'm heading back to the Wii Fit again.  That bike is starting to scare me.  I never thought I'd have to check the barometric pressure just to ride an exercise bike.

So when I lose 50 lbs (and I've lost 14 that I know of already), I'm buying myself a really nice bicycle.  I hope I live through the stationary one to ride the real one and then I hope I live through the real one too!