Hi Dr. Andrew, I Had Sex For The First Time

Hi Dr. Andrew,
I had sex for the first time on Saturday night and didn't use any protection as I am on the Pill. Sunday morning I realised I had bled, it was more of a brown blood rather then bright red. I had sex again last night and the same thing happened, except this time there was more brown blood. My question is, what are the chances I am pregnant? Does this sound like my Pill isn't working or am I worrying for nothing?