How do you know if it's a miscarriage?

I recently stopped depo because my boyfriend and I broke up and then need to prevent pregnancy walked out with him. I had my last shot about 4 1/2 months ago. I never had a period in the 2-3 years I was on it and I expected my first one to be pretty bad. I've been bleeding so much though and these cramps are not like I've ever had before.