Oral Health Question?

I have a bad overbite, sometime near Christmas 2010 I got braces on my upper teeth. Recently I've had them removed. But when I had my braces on they pushed my top teeth back, like they should have..but with my overbite I tend to bite the roof of my mouth, especially when I sleep since I clench my teeth. This has developed what feels like a loose mass that I can move with my tongue on the roof of my mouth just behind my front teeth. My dentist in the process of removing my top braces ripped the mass causing it to bleed quite a bit. It may have to be cut out if it doesnt correct itself. What I was wondering is..what is it exactly?