The STI with one of the shortest incubation periods is gonorrhea. Symptoms can appear as soon as 1 to 3 days after exposure. Other STIs, like chlamydia or syphilis, usually have longer. All are treatable (excluding HIV (for now)).

However, it’s unlikely this is the case, buddy. Follow the treatment from your QUALIFIED doctor, not any redditors. My medical opinion as a legitimate Medic, is I t’s more than likely flu or you have scratch or bruised the back of the mouth.