Sudden reddish, somewhat itchy rash around groin area, what do I do?

Don't panic. Nothing makes people more nervous than a rash in the genital area. It could be anything from jock itch to an allergy to something you applied in that area to, yes, something you caught from intimate contact. Put your mind at ease by seeing your doctor and getting it sorted out. You can try otc Hydrocortisone cream first if you wish, but if that doesn't work, see your doc or a dermatologist.