Brown discharge after sex?

I am on the pill 'ovranette' and missed one a few days ago and took yesterdays one a lot later than i should've. However, with this pill i should still be protected from pregnancy until i miss 2/3 pills. I had unprotected sex with my boyfriend today. About half an hour later i noticed i had a lot of brown discharge but i cant really tell if its blood or not. Could i be pregnant? But surely if i had got pregnant today it would have taken longer than just half an hour for this brown discharge to come?Ive had sex a few other times since my last period but never got this, this is the first time so i dont think its old blood. My next periods not due for another 7-10 days. Could i be pregnant or could it be something else? Thanks in advance.