Cause of reoccurent sores/pain in mouth?

Last year started all my mouth problems. I had severe pain ( everything swollen in my mouth, a tongue that resembled thrush which then started to crack on the top later, I had canker sores etc. Doctors were baffled what I had. My lab results came back with no unusual findings. I took meds for it and it cleared. It came back about a month later. A nurse said the previous meds didnt kick the infection out, so she gave me stronger meds. It went away. To this day though (a year later) I get random sores in my mouth every few months or so that go away in about a week. I have some right now. It started out looking like a canker sore on my cheek near my gum line in front (it had a white spot). Today I have pain under my tongue where the tongue is connected the base of my mouth (behind my front teeth). The area feels inflamed and sore. A little about me-- Im 24, have good brushing/flossing/fluoride habits, am a non smoker, takes no illegal drugs etc. Does anyone have any reason for my pain??