Never been to this site before, and not sure if anyone still comes here, but I need the always reliable help of the internet for a bit of a pre-diagnosis before I had out to the dentist. I'm 14, my birthday is literally tomorrow (Halloween) so I guess you could say I'm 15. Recently, I've noticed a rather large, painful red gash underneath my tongue. It's on the left side of my mouth, behind and under my tongue, and down low, almost near the floor of my mouth. I've tried vigorously to view it in a mirror, but it's way down there. Trust me though, I know it's there, and by f*****g god does it hurt. I'm just wondering if this is just a canker sore, or something more serious.