How come only some days I can't ride my bike for very long - it exhausts me?

Please read this before you answer. A lot of times I can ride my bike for 45 minutes until I get tired. Then, randomly, some days I go for a bike ride and am tired in the first 10 mintutes, then from there I get exhausted, drenched in sweat, and to the point where I want to cry. It is so weird! Normally I am fine. Today I only lasted 20 minutes and I feel awful now, I usually feel great after a bike ride. Please help :/
 
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