Quote Originally Posted by zencentury View Post
The uterus can have muscular contractions, I would compare it to any other muscle with an imbalance of electrolytes causing cramping/contractions.

Only on a hill climb? It may be related to the muscles you're using at that time, i.e. you position on the bike.
My vote goes to the usual suspects, hydration and electrolytes. If you can find a flavor you like try mixing a 50%solution of gatorade instead of just water for those hot days.
Seconding this theory. Electrolyte deficiencies can definitely cause or worsen uterine cramps. That's why taking calcium during your period can ease cramping.

The fact that this mainly happens to you when it's hot seems to fit this theory. If you had these cramps no matter the temperature, I would be more inclined to think it is a physical irritation problem (too much jostling, too much strain on the abdominals, etc).