So here's my question, I'm a little embarrassed by it but I've been driving myself crazy thinking about it and really need some answers. So I've been with my partner for about 5 years now and have never had any other partners in that time. This past Friday/early Saturday I have a little too much to drink and had a breif sexual encounter with someone else. Since then I've been feeling really guilty and bad about it and have been roaming the internet looking for answers, etc. Here are the facts so far:

There was some brief touching of genitals without a condom and one insert and out, rest of the time a condom was used and I stopped it after a couple of mintues. About a day later I began feeling like the top left of my vagina (or rather where the top skin starts) was sore (could be because it was rough and he was bigger than what I am normally used to). The soreness also felt like it was moving toward the left groin (could be because I read stuff on the internet saying a symptom of STD was enlarged lymph nodes). I don't feel anything large bumps or anything. Day 3 I started having some gas like cramping and a slight brownish vaginal discharge. The discharge does not have a bad odor (smells like regular discharge) and I'm not itchy down there nor does it hurt when I pee.

So here's my question, most places I've read say that it's very unlikely to start having symptoms of an STD in 2 days but other places say yes it can. I'm really going crazy right now thinking about this and reall scared at the moment. for the most part nthe gas cramping has gone away but I still feel like there is some pressure (not painful) in my left pelvic, lower abdominal, groin region.

Also, I'm expecting my period next week so not sure if this has anything to do with it either.
Could it be possibly that I have an STD like Ghonnorea or something, or acould it be that I am stressing myself out with the guilt and this could be causing some stuff also? Any insight would be much appreciated.