No period For 16 weeks

Hi, I'm hoping you can help me. I had the depo injection 16 weeks ago and in the 12 weeks that I was told it lasts I did not get my period at all. Since going off the injection almost 4 weeks ago my nipples are sore and when my belly is pushed on it kind of hurts. I wanted to get the depo shot once my period returned but it still havent and I'm worried I might be pregnant. Please help..