My Question Is What Could Be Stopping Me From Getting

My question is what could be stopping me from getting pregnant? I'm 21 years old and I had a miscarriage when I was 14 and have not been able to conceive ever since. I was on depo shot when I got pregnant as well and after that the doctors gave me a double dosage. After I got off the shot I continued to bleed for weeks until I had to get on the pill to regulate everything back to normal. I have regular periods and they're only painful on the first day usually. Please help me as I'm so confused.