What Causes Small Round Open Bleeding Sore In The Back Of Mouth?

I have a small round open sore in the back of my mouth, from looking at diagrams I'm pretty sure it's the retromolar trigone. I've had pain in this area of my mouth before but it's always been just sensitivity when eating, sharp pains when some hard food accidentally crunched down on it. But this latest time there's a small open sore that has been bleeding. I always use kanka medicine to numb the pain and before the pains always gone away after a short time. But his seems to be lasting longer than normal and not getting better. I haven't been feeling 100% healthy either and don't know if its related.