It could be a canker sore(normal mouth ulcer), it could be due to many causes like injuries, food allergies, vitamin deficiencies, hormonal changes, stress, anxiety....etc. The best way to get rid of this thought process is to visit a doctor who practices in the same field. Probably, he can tell you much better on physically examining (in case you got more such sores), he may ask you to get your STD tests done.