Hello and welcome to the boards.
From what you are describing, this sounds like a mild Herpes outbreak. I don't mean to scare you but if this is something that keeps coming back then is sound s like something of a herpes nature. Most liekly this sounds like HSV-1. If you have practiced anything orally, then it is possible to pass HSV-1 which is also known as cold sores or fever blisters.
My advice would be to see your doctor and ask for a Herpes specific test. This is a blood test that can determine if your body has any antibodies for HSV1 or HSV2. Keep in mind, it doesn't mean your girl cheated because people can carry Herpes for years before anything shows.
Now here's another option. You may be allergic to a type of soap or detergent you may be using. This is known as contact dermatitis and many guys have this. Mine had it once because he's allergic to Irish Springs Soap. ALso it could be jock itch and Jock itch is fungal.
Just make an appointment to see your doctor. There could be lots of things going on. If there are any questions, just ask away.