Hi there ~
I am sorry that you are having problems with irregular periods
. If you are sexually active I would suggest that you take a urine pregnancy
test. You might be having threatened abortion
which may be infected and you might have brown discharge. However, this kind of discharge may be due to many other reasons. I would say that the back ache is probably due to your having some sort of infection in your pelvic area, however this may have to be examined by a gynecologist. An abdominal ultrasound
might also suggest the cause for your problems.
I hope this helps. Take care and have a lovely day !