Implantation Bleeding???

I'm currently trying to avoid pregnancy because I'm only 17 but I have recently began having a sexual relationship with the guy I'm talking to. We have known each other for 4 years & have been on and off in those years. But only recently have we actually started a serious relationship & begun having sex. I am on birth control but stopped taking it a few days ago when I first suspected I might be pregnant because I was afraid the birth control may harm the baby if I am pregnant. We never use a condom when we have sex. Today I noticed some blood when I used the restroom this morning. Then I headed over to my job, I currently babysit. After a few hours I got up to go use the restroom again & noticed this time it was a lot darker, like a brown color. But it was a light pinkish color in the toilet. We had unprotected sex Tuesday the 13th. Could this be implantation bleeding?