Red rash on groin day after sex - could this be an std?

Probably not: No STD can start to cause symptoms only one day after exposure; 2-3 days is the minimum. And most rashes in the groin are not caused by STD. A bacterial or fungal (yeast) skin infection probably is more likely. If the rash doesn't clear up within a few days, see a doctor. But don't worry about STD; whatever you have, it isn't from your sex partner.