It could be that due to sweat,your feet are slipping inside the boot. Try a thin insole,to see if that helps.
Probably cramps. I get them when I play and when I train, but I still don't know how to fix it, so I usually just suck it up and keep playing
I think Bear is onto something there about your feet slipping and friction but I wouldn't go for a thin insole. I was going to suggest thick socks (to counter movement and to soak up sweat) but more importantly I would suggest Sorbothane insoles. Try different thickness socks in training to find what works best but really the Sorbothane insoles can change your life. They can be quite thick so might make the thick socks impractical. Try find what feels best for your particular boots but if you find the full insole too bulky, then at least get Sorbothane heel inserts. I first used them when I got plantar fasciitis and I swear by them now!!
