can an std come back?

 I went to the doctors and got a shot and a single does of two pills. the only symptom i had back then was that i had extremely painful ulcers in my throat. well that was about 3 months ago and he told me to go for a follow up but it all went away and i felt perfectly fine and now 3months later, its back but a lot less painfull its more of a discomfort unless i eat certain foods. I looked at my throat with a flashlight and i only see 2 not too big last time it was like multiple ones and all over the back of my throat. ive been taking tynenol just for the pain and this tablet that dissolves in your mouth too, today the pain has gone down even more so im just wondering what it is because i have no other symptoms of anything, just in the morning my nose is a little congested but nothing too bad. last thing when i went to the doctors he told me wait a week to see results so im a little confused as to how he knew how to treat me if they didnt exactly know what was wrong, i had told them i had oral sex though and after he gave me the medicine and shot it went away soo. sorry if this is long thanks.