Spotting brown blood for three days...

Drea
I'm not on birth control, but my bf and I are having unprotected sex. We're financially stable and all that good stuff. Anyways. I went to Hawaii June 4-12 got my "period" but it was just really light brown only spotting from the 6-9, like barely touched the tampon I put in. Why would my "period" do this? The only thing I can think of that would change my period so drastically is either running or pregnancy. Which one is more realistic?