What Causes Vaginal Spotting While On Depo Provera Injections?

Hi, I was on the depo provera shot for about 6 months( the whole time i had no period or spotting.)I had my last shot feb 2014 I started to bleed end part of july it was mainly just brown mixed with red spotting spratically for 3 weeks (im thinking its my period trying to come back ) on the fourth week I started a red light flow period that was more than spotting and consistent threw the week. Ive hadnt had any type of spotting of flow for about a week now. i had unprotected sex the end of my fourth week of bleeding. And then once the bleeding stopped ive had sex every day this week. BUT tonight when I went to wipe I had a little bit of brown blood and nothing else so far. Could I be pregnant and its implantation bleeding or do you think its my period still trying to come back and too much sex is preventing it?