Hi symphony,
I am guessing since it was in May that you posted, your viral has all ready came and gone but for anyone else who needs information:
The doctors right, usually when it happens "overnight" or "all of a sudden one day" it usually just means it is a viral infection. I actually have the same thing you had. its my first time getting it but my little ones get it all the time when cold season comes around. Its not bone tissue, or cancer, or herpes-usually-
Its just a viral, you dont really need to go to the doctor for a viral because usually what they will do is say "yep you have a viral call us back if it doesnt go anyway in about a week so we can set up another appointment" what? there goes 200.00 down the drain just to tell me if I have a viral! lol just some advice
If the lesions are acute onset, multiple discrete vesicles, viral infection is suspected. Seeing an oral pathologist or ENT is advised.