Sex two days ago now diarrhea why ?

Maybe unrelated: This may have nothing to do with that sexual event. No STD is likely to cause diarrhea only 2 days later. If your partner had an intestinal infection, you could have caught it; being close enough for sex is obviously close enough to catch an intestinal infection. Or from an entirely separate exposure 2-5 days earlier. (If you have a young child at home, s/he probably is the most likely source.)

No relation: no relation except stress, if for any reason you are having more stress than usual that could explain the diarrhea, but not the actual sex.