Welcome to healthcare Magic!
Lower backache and frequent urination
can be pelvic inflammatory disease
secondary to a low grade chronic inflammation due to an infection.
can be a possibility but much less likely because it mostly presents with post coital/vaginal bleed.
Still a per-speculum examination and high vaginal swab
and Pap smear would help making the proper diagnosis.
Please get these investigations and if you like, inform me in a Fresh Query / Follow up Query for further fine tuned opinion
Wish you a great health!