'HI' reading after hitting the gym

I'm really confused! I've just about recovered from a reading so high last night that it went off the scale for my meter (>30 mmol). I had been to the gym to do a class which I do every week. It had been 9.7, for over an hour before the class-not ideal but not high enough not to exercise.
I had no dextrose before the class-I test half way through and respond then. It rose to 13.7, half way through the class! I know this isn't unusual and that intense exercise can cause BS to rise-and mine often do with intense exercise.
I felt so ill by the end of the hour class. I thought I had just burned out after such a hard class. I could harly move my arms to drink! I stupidly came home and cooked. It wasn't until an hour later that I tested again-expecting a drop in BS or it to have stabilised.
I was mortified with the 'HI' reading. It hasn't done that for years-10 or more.
I treated the high with extra insulin. I ended up taking an extra 17 units. It has taken ALL night for it to fall back down into range.
WHY?
Has anyone ever suffered with such an extreme reaction? I've checked my readings and I've noticed that I often get high readings after my gym sessions-but not always making it very hard to predict.
:?

Thanks for the advice!

I was thinking about possibly taking insulin before hitting the gym-but I'm still a little worried about the times my BS actually drop. I'm going to monitor it rigidly over the next week.
What a pain in the neck, eh? You go and exercise to help control your condition and end up making things worse!

Has anyone else been adivised or takes insulin before exercise?