Why am I bleeding in between periods?

Katherine

I missed 2 periods a couple months ago(I could not be pregnant). I got it back in June but it was 5 days early. Then, I had sex on July 2nd and got my period on July 4th. I had sex again on July 10th. Since a couple days ago, I've been spotting brown in my underwear but no fresh blood. I haven't had any pain at all. What's happened?

Answers much obliged!