Thanks for posting,
When you experience pelvic pain/lower abdominal pain
associated more or less to lower back pain
it is usually as a result of an affection at the level of your pelvis or kidneys/UT. Now first of all, since you have missed period, take a pregnancy
test, and if positive consult a gynobs so he can run a pelvic scan to determine if any abnormal pregnancy. Some series of tests include screening for PID (which is very likely given the presentation) and also urine tests + more or less of some blood tests could determine the cause of your problem and henceforth an appropriate treatment can be established.
Hope this helps