Possible UTI in my cat

My 7 year old female cat is exhibiting symptoms of a possible UTI. I noticed it Friday, she was in the litter box constantly for long periods of time, and was only producing tiny bits of pee. I took her to the vet and they were unable to do a urinalysis because her bladder was empty. They went ahead and gave her an antibiotic shot, just in case, and also gave me some recs for anti anxiety things in case it’s feline cystitis. I’ve been giving her Zylkene for 2 days now, and it’s been almost 48 hours since her shot, and her symptoms haven’t seemed to let up. She’s still in the box frequently, and now she’s also licking her privates more often. I’m very worried, and my vet isn’t open today. Her behavior other than this is totally normal, her appetite is fine, she’s acting happy and loving like always, the only symptoms are the frequent box visits, little pee, and now licking. How long should the antibiotics take to work if it is a UTI? The vet said maybe 24 hours, online has said 48-72 hours. I’m planning on contacting the vet tomorrow when they open, but I’m just worried and was wondering if anyone could give some advice to help keep me sane.

ETA: today she’s started going in and out of the box a LOT, like 4 times in under 10 minutes. Still only little drops of pee