As it is her back, I am suspicious of a flea allergy.
While I appreciate she has had bladder issues, as she is currently not showing signs of being affected and is not over grooming her tummy, a flare up of this condition is less likely.
Unfortunately many fleas are now resistant to the active ingredient in Frontline (Fipronil). If she does have a flea allergy, even a single flea will cause a flare.
Snoppy must be treated with a strong de flea treatment. Advantage is the best OTC product in my experience. Advocate can be purchased via your vet, which is a similar product.
After applying spot-ons, animals must remain dry for 2 days so no baths or swimming.
Once she has been treated, we need to move on to the environment. This has to be done on the same day. While 5% of the flea burden live on the cat, the remaining 95% are in the bedding, sofas etc. We need to hot wash bedding over 60'c, hoover throughly and use a flea spray that contains and insect growth regulator such as Indorex or RIP Fleas to eliminate the issue.
Sometimes this needs to be repeated a few weeks later in case any eggs/larvae have escaped the initial cleaning.
She must be treated for fleas every 4 weeks.
It's important to say that fleas carry tapeworm so ensure her tapeworm treatment is up to date.
If itching persists, she may need an anti itch injection and some antibiotics from her vet to break the itch scratch cycle and allow her skin to settle.