Birth control and pregnancy question LADIES PLEASE HELP?

I have had the birth Control tube in my arm for almost 2 years. its suppost to be VERY affective. I stopped Having a period all together. I used to have unprotectec sex every now and then and everything was fine. I stopped for about a year. and recently I have been having A LOT of.unprotected sex with my bf. (2 times a day, every couple days) ..... for about 2 weeks. and still I never usually get a period or bleed at all. but a few days ago I noticed some bleeding that was dark red/actually more brown and I found out that's something that usually happens about 6 to 12 days after a baby is concieved.. and I've been feeling a bit bloated latley but not much different. I have heard of only a few people getting pregnant on this type of birth control.. I am just wondering how possible it is that the bleeding was Infact. implantation bleeding and I could be pregnant. someone help!!