spotting on the pill

Hey girls I'm on the pill but I'm also in my fertile window. Yesterday I noticed some light brown blood on my pad. It wasn't much but still I'm a little freaked out. 
My boyfriend and I had sex two days before my fertile window started. We users condoms and he pulled out. I took the pill religiously at the same time everyday. I don't know what to do cause I've read it can mean many things so I don't know what to thing. Help?