I was so stupid, I let my friends talk me into doing this and now I have an ugly purple burn... it hurts too! Later I began to read it was frostbite and I got scared, but then I came across someone on the Internet that said I can cause cancer? So can it?

Cancer, highly unlikely, although it WILL hurt for a while

I don't think it can because its only ice under your skin. But I did it to I have to ugly marks I'm also scared hope find a cure of some sort. Oh and I heard tooth paste cure the under the skin affect

The only "cure" is time for healing.
Hopefully you have learnt just how dumb a stunt like this is..