First, i would like to mention that the pain killer which you took for toothache ideally can not cause lower back pain
. What i feel is that you may have pulled a muscle at the lower back and you may also be feeling discomfort in lower ribcage area due to medicine on empty stomac.
I would suggest you to locally apply an anti inflammatory analgesic
drug spray to ease the pain and also to have a glass of cold
milk if you have acidity and discomfort in lower ribcage area.
If the pain persists then you must visit an orthopedic surgeon or your physicians for the same.