2nd period?

Yesterday I felt really sick all day. I stayed in bed hoping I'd feel better at some point. That night I went to get into the shower and noticed that I had bled a little spot in my underwear. I thought it was weird because I had already been off my period for a week.
At first I thought it was just spotting, but then the bleeding got heavier (kind of like the bleeding of one of the last days of a period).
I'm not sure what's wrong. I'm on birth control. The doctor put me on it because I was having two periods in a month. It stopped after I started taking birth control, so I don't know why it randomly started again.
Could it be anything else? I'm a little worried and I don't want to have to pay to go to the doctor if it's just a second period.