Need advice!

I got my first birth control depo shot on my daughters 2 month appointment, May 7, 2014. My boyfriend & I had recently had sex, he came inside me, and I feel like the birth control isn't going to work. I'm thinking my last period was July 15,2014 but I'm not for sure because when I got the shot I spotted a lot, but on the 15th it was heavy. Any advice on the birth contort shot?