"mini period" mid-cycle. what could be the cause?

I'm 25 years old. I've had my period since I was ten and never had mid cycle spotting before. It reminds me a lot of the last day of a period. It's mostly brown or pinkish and doesn't require a tampon although I do use a tampon. Today is the fourth day of this. The first day there was very mild cramping, but nothing since. I had sex 2 days, 3 days, and 13 days before the spotting started. It was unprotected, but he did pull out. I'm not on any sort of birth control. My last period ended 12 days before the spitting started and I'm not due for another period for seventeen more days. I've never had anything like this before so it has me a little worried. Any ideas??

Please and thank you!!