good evening to you...you seem to be experiencing menstrual irregularities...lower back ache is quite commonly seen in women...start taking calcium supplements
...eat diet containing milk...if you are a veggie try eating drum sticks ,known to be a good source of calcium....get your hemoglobin
checked and rule out anemia as it could also be a reason for your extreme tiredness
...eat lots of dates,almonds...dont panic regarding your menses..some kind of stress can also cause such irregularities at times....wait for a month...if it keeps happening again then you can get your pelvic ultrasound
done and get yourself examined by a gynecologist...