may I please have some advice?
 

I'm 21 years old, and around 4 days ago, I started bleeding really lightly, like enough to only be on the toilet paper. While that would have been two weeks early, I still figured it was my period. It ended up only lasting a little over a day, and was always brownish in color, and never more than spotting. I also didn't have my typical PMS symptoms beforehand.
I have had unprotected sex for well over a year, and I haven't gotten pregnant, so I'm sure that can't be.
I did start taking medicine for my ADHD recently for my classes, but it didn't effect my last period at all, and I only take the stuff two times per week.
Does anyone know what this could be, or if I should be concerned?