Urinary problems?

A little backstory- this all started in July when I took my cat (3 year old ginger tabby girl)  into the vet for a UTI, thought all was well and good after no accidents, etc. Until last month when she started urinating inappropriately again.  To the vet we go.... again. Another UTI, we go through the antibiotic shot again (no pills, or oral meds as neither ends well).  It has now been about 3 weeks and she has started urinating outside the box again (on my bed, on clothes, on the floor). I don't want to go back to the vet, but at the same time what if it's another UTI.  We also live with two other cats that she does not get along with and I'm not sure if she's inappropirately urinating because of them.

Our little girl cat had a UTI that the Convenia antibiotic shot did NOT take care of in one shot.  It took a couple of injections to fix the issue.  Our Vet mentioned that Convenia isn't really the antibiotic of choice for a UTI, but, like you, pilling wasn't a viable option, so that's what we went with. 

Unless the other cats are new to the household, I wouldn't think this is  a territorial thing, but are you making sure you clean up all her messes with an enzymatic cleaner.  Just tossing everything in the washer, no matter how hot the water is, will NOT get rid of the scent to a cat .  They will be prone to return to that area again and again. 

Stress is a contributing factor for UTI.  Has the vet done a urine culture to see if there's something else going on?  Has the vet ruled out crystals?  Do you use any calming agents such as Feliway or Rescue Remedy?  Is she on a urinary diet?

Thanks guys! Talked to the vet, they said her urine pH was a little high, and that the next course of action would be placing her on a urinary diet, so seeing how that one goes.  She's a pretty laid back kitty, so the only time she gets super stressed out is when she has to go in her carrier. Fingers crossed her new food will work!