What Causes Bump On The Inner Wrist?

Okay, this may sound weird, but I was sitting and doing my home work when suddenly my left inner wrist started to itch a bit, then a bit more then a lot more. I wrapped it to prevent it from rubbing on my text book, I unwrapped it minutes later because it was irritating me and there was a bump, it wasn't red just a bump. It looks like a cyst bump [I just j=had one removed from my other wrist]. It happened so suddenly, can anyone explain. I'm 17 and female.