I actually had this problem as well. Mine did show up after sex, but it wasn't an STI/STD. Though, in your case it just might be, but the STI/STD is treatable and curable, so please don't freak out. The other option that it might be is Prostatitis. And what that is, is the enlarged inflamed form of your prostate that is squeezing the tube that sends it's fluids to your semen to help semen come out of your penis. It's not really exactly known how people get it, but I got it three times, twice from jacking off while on the meds (yeah, I know..my bad). Sometimes a disease can be contracted if you have unprotected sex as well, so make sure you ask all questions and make sure everybody is clean inside and out. You will need to go to the doctor. The procedure will be that they will check your balls first, that's sort of standard anyways, but there is a disease that can cause this just from your balls. Next they'll stick a swab into the tip of your penis, to try and get a sample of the discharge, then go ask you to take a urine sample, so go pee in a cup. They may need to take a blood sample as well, even though AIDS/HIV does not cause these symptoms to show up, another reason to not freak out. The test results will come back later, but they'll give you a form of anti-biotics, and YOU NEED TO TAKE ALL OF THEM, and please do not jerk off during the time you are on them, it'll mess things up majorly. But please, go see the doctor, because what you need to freak out over is that if you let this keep going it can majorly mess up your reproduction organs, even lead to causing cancer as well. So go very soon please.