As our members have told you clearly, you had no risks, we are sure your test will be negative. We wish you the best.
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You say he ejaculated on your shorts - did you have your underwear or any clothing on when this frottage happened? If so, was your anus covered?
If your anus covered, there's no risk for anything. Taking prep was not necessary, clothed or not.
So I'm a little confused here - was this intercrural sex, also known as thigh sex, where his penis was in between your thighs, and it was like non-penetrative sex? Or was this just some rubbing of his penis between your thighs and near the anus for a bit? How long did this last? Was lube used?
I ask because there may be a risk for HPV, herpes or syphilis, but the risk is probably really low because the skin on your thighs is too thick for anything to penetrate, and if lube or lotion was used, that would ease the friction, lowering the risk.
You did nothing that would give you anything that would cause discharge, and no STD causes diarrhea. That and the stomach cramping could be side effects of the prep, though.
If the discharge is clear, that could be normal.
I wouldn't overly worry about this. If your penis wasn't inserted into anything, and no one inserted their penis into you, the chances of getting anything are very low, or zero, depending on the infection.