I have a push mower as well and if I go do it in the heat I suffer later and I'm reasonably healthy these days.

My personal theory and here's the short version is that the combination of the over exertion and the heat increases lactic acid and with increased lactic acid comes the anxiety and panic. With prolonged exercize I sometimes get this strange jittery feeling all over and it feels as if I'm jittery inside my body as well.

If I were you I'd take some magnesium, drink lots of water, and take it easy today so that you can recover.

I hope you're feeling better later.