Breakthrough bleading after . Ive been having problems?

I've had two ultrasounds on my ovaries because i was having pains and it came out ok. They said that i did have a small cystic formation. My Nurse practioner said that it was probably due to having a l infection, which she treated. It's been a month now and I haven't had any problems until I had ual . We haven't had since I was having the problems. Well, after having I started lightly bleeding again. What could this be?