How To Treat Back Pain And Burning Sensation During Urination?

im a 32 year old woman...i have been having dark brown discharge before my period in 2 weeks...it seems like its old blood. and in addition about a month ago i had sever back pain followed by aburning sensation when urinating which i tried to heal by changing my diet but i finaly statrtd taking anitbiotic 3 days ago and it healed it but i feel maybe there is a little more sensation and should complete a 5 day course instead?