usually this is a blocked oil gland. be seen today, don't put it off.
meanwhile you can do something as simple as sitz baths or warm compresses to the area until you are seen.
since you had unprotected sex, best to cover your bases with std testing at the proper times. since this isn't a long term monogamous partner, you shouldn't be trusting of his std status since he's obviously willing to have unprotected sex. when to test for what is covered in our read before posting post on the forum.
grace
Hi and welcome. You would not get an std symptom the next day. Symptoms are usully a white/yellow discharge or burning when urinating.
Maybe it happened from the act itself like an irritation that got inflamed by the next day or an inflamed hair folical.
However, if you have had an active sex life before this with other people, you might want to have some testing done while your at the doctors to make sure all is ok