I have severe lower back pain, and i'm 2 weeks late on my period but i took a blood test thatcame back negative. i'm not sure what's wrong ?

See your doctor: You should see your doctor, given the severity of your back pain coupled with the fact that your late.

Several possiblities: Late periods are always of concern & should be evaluated by a doctor.I recommend an osteopathic dr as they can not only do all the tests/exams your md can do, but many are also trained in osteopathic diagnosis, hands-on work which can help find subtle causes of both mild & severe pain.If cause is found, osteopathic manipulative treatment right in office may fix the problem.