I Had My First Depo Shot (this Round) On March

I had my first depo shot (this round) on March 18, 2015. I didn't follow up with anymore so I only had that one. My periods stopped. My first period came back around July 6, 2015. It lasted about 4 days. Around july 18, I started my period again!! So I had 2 periods in one month. I was unsure when to expect my next period. August 14, I took a pregnancy test. It was negative. I wait for my period but it never came. On September 22, I took another pregnancy test. It was positive. My question is can u help me pinpoint a time frame that I was ovulating? Or what the possible dates of conception were?