If after thorough check up in ER nothing has been found contributory to any obvious pathology, it is worth wait and watch giving her complete bed rest on firm mattress, hot packs and some pain relievers (must have been ordered in ER).
your daughter is pretty young to get primary back pain
so spinal disease
is unlikely.it may be some hormonal
imbalance possible with history of ovarian cysts and some times it may be due to UTI itself (uretric inflammatory pain- tubes that drain urine from kidney are infected )
If possible get her checked up by a gynecologist/ urologist