Pregnant or innocent bleeding?

Hi everyone,
I had my period last week, and since this morning I've been experiencing red/brown bleeding. It's less heavy than my normal period. I'm 20 years old, not on the pill, and sexually active with my boyfriend. Before my last period started, I took a pregnancy test, which was negative, and a day later I got my period. It lasted about four days with heavy bright red bleeding. Last Monday it finally stopped. Has anyone else experienced this? Bleeding in between periods for no apparent reason, etc.?