I had a similar saddle sore. It would go away between rides but then pop back up after a ride. My doctor prescribed an anti-bacterial cream, which helped immensely and I thought it had gone. Then after a particularly long ride, it popped back up again. Then the doctor prescribed a course of antibiotics and the problem appears to be gone forever.

So, if I were you I'd talk to my doctor and get it fixed once and for all.