Thank you for your query at DoctorSpring.com
With a previous history of UTI, and current dull aching pain in the back, it is mostly related to your UTI.
You need to start antibiotic right away, as the fever and generalised discomfort associated with UTI can be severe if not controlled early. Regarding spreading to kidneys, chances are less, it depends on how you respond to the antibiotic therapy.
Kindly get a prescription for Tab. Cotrimoxazole ( TMP-SMX ) which works really well for UTIs. It needs to be taken for 10-14 days depending on the severity. Also you need to drink plenty of water and cranberry juice. Stop having vitamin c as it will make the urine alkaline and cause more discomfort.
Hope this was helpful,