What Causes Back Pain With Delayed Periods?

Hi, I have a question I haven't had my period in 2 1/2 months. I took two pregnancy test and it had said negetive. I went to the doctor and they took blood work and they said I wasn't. After a week I'm getting really lower back pain and very bad cramping. But now I don't know what it could be so what could it be? I'm getting frustrated