please reply!! brown discharge instead of period?

Okay so I'm on the pill And take it on time and properly all the time.
But my last period lasted about 4 days and it was only brown, no red or blood.. is that just a symptom of the pill? 
I have unprotected sex with my boyfriend and this type of thing has happened to me before and I got my period normally like not brown, but I was just wondering is it a normal symptom for the pill? 
Thanks girls xx