This sounds most like tinea cruris (jock itch). Definitely not an STD; the symptoms don't sound like STD and 8 months is much too long for any STD to show up.
This has nothing to do with the sexual exposure you describe. Try an over-the-counter antifungal medication (look in the pharmacy section where the athletes foot products are). If that doesn't help, see a health care provider.
Good luck-- HHH, MD