So I started the pill when I was on my period (I was told if you start it on your period it works…

So I started the pill when I was on my period (I was told if you start it on your period it works immediately). 5 days after that I had unprotected sex with my boyfriend. I was supposed to have my period last week. I took a test before I was supposed to have my period and after. Both negative. Well a couple days ago I had light brownish spotting. Only lasted two days. Do you think my period is out of whack due to the birth control or could I be pregnant? I'm 3 months pp.