Could I be pregnant? I started depo after my period

Hello, I m very concerned right now. My lmp was 7/31/2011 It lasted until 8/3 and it was regular. I decided to take depo my doctor gave it to me on the 8/10. I immediately started having unprotected sex with my mate the next day. My period is now 8 days late and after doing tons of research I have found that if you don t take depo at the time of your menstrual, it takes about two weeks to work. This means that During my fertile period 8/13-8/18 when I was having sex with my mate, I wasn t protected by the shot at all. I have been on depo twice before a few Years ago and it has NEVER made me miss a period. Although I have taken 3 home pregnancy test and they have come up negative but my symptoms are lower abdominal cramps(feels like menstrual is coming), frequent urination, exhaustion, mild Pain in my right side of my abdomen, larger appetite, nipple are a little sore, and mild headaches. Could I still be pregnant but for some reason the home test aren t picking it up?