I had something similar earlier this year.
I got strep in Jan, took the antibiotics, it went away. I got it again in Feb, they gave me antibiotics, I went on Spring Break. I felt better for awhile, got lax about taking them and on the way home, got sick again. They gave me a stronger AB, but it didn't do anything. I went to the hospital, they did a blood test and I had a pretty mild (All things considered) case of Mono. I felt like shit for about a week, slept it off and started to recover. I felt fine and then I started to feel strep-y again, this is in mid-Aprilish. Nothing seemed to be able to shake it. In late May, I returned to the doctor's, got more ABs and they finally stuck. I haven't been sick since, so go figure.