What Could Cause Spotting?

I'm 17 and I've been on the same birth control for a year then i switched to a new one and then i stopped taking that one for about a month because it started to break me out. Then i got on the new one in I'm in the first month and my period came September 18 and last 6 days but i spotted on the 16 but it stopped and my period came on the 18 but then on Monday night i had some cramping followed by light pink spotting then this morning it was brown and this afternoon it's gone.I'm sexually active and only had sex with one person and we are both disease free and we use the withdraw method. why am i spotting this never happen to me before.