sounds like an ulcer to me
my SIL in pregnancy developed an ulcer on her leg and one on her tongue (by the end of her pregnancy the one on her tongue was the size of a 50p piece and about a cm thick and she had to have it surgically removed) - she was told that it happened to some women in pregnancy
the only reason I would think it wasn't an ulcer is because a dentist should recognise it quite quickly
get it checked out