Without knowing what you've taken in during and after your ride, it sounds like your electrolyte balance is probably off. It's hotter than hell (literally) and you sweat a lot (at least I do). You lose tons of electrolytes in your sweat (primarily sodium) and if you take in only water you're not replacing everything that's been lost. I only drink water on recovery rides this time of year. Anything over 1 hour and anything over a recovery effort, I'm drinking Gatorade.
It could be some kind of weird muscle pull too. I've had those kind of involuntary twitches before too, with no explanation as to why it happened.