Spayed Cat in Heat?

Question
About three weeks ago, I found a little kitten freezing in the cold outside of where I work on a Friday night with a bunch of mucus frozen around her nose.
I put her in a box and tried to find a place for her to go because I couldn't let her on her own out there, but it was a Friday and Animal Control wasn't open, the shelters weren't open when I was off work/have 2-3 week appointment waits and I had no other options. My boyfriend decided that because she had the top of her ear cut off, she was more than likely feral and spayed and vaccinated, but she was so social and snuggly that I had trouble believing she was feral.
We took her to a very low-cost clinic, as recommended by one of the 24-hour vets I called, and they were able to give me a possible age as determined by her lack of teeth and major tartar, 12, and her weight, about 3 pounds. They also gave her some Clamavox for the URI she had. That only lasted 5 days, and the infection did not go away. She had not been eating until the Clamavox and then she had been ravenous, not stopping whining for food at all hours of the day, especially for her size.
I am in college and she is illegally living in my boyfriend's dorm room so it took me until my next paycheck to get her back to a new vet for more antibiotics. When I could get her back, they gave her an injectable antibiotic and after a few days, she was better. Herein lies the problem: she seems to be going through heat: excessive howling, cleaning herself, rolling on the ground, backside in the air, the first being the absolute worst in a dorm setting where my boyfriend can be pretty severely penalized for keeping her. Since I was told she was spayed, hence the ear tagging, but she is definitely exhibiting signs like she is not in heat.
Is it possible that they did the surgery in a cost-effective way that did not involve removing the uterus? Or could it be behavior changes due to possible hyperthyroidism, as determined by the excessive eating and not gaining much weight?
Thank you so much for your help! We're first time pet owners, in not the best situation.

Answer
Hi Daisy,

Thanks for doing this good deed. An ear tip would seem to be evidence enough that she's spayed, but the vet should be able to feel some scar tissue at the incision site, as well, to confirm.

If confirmed spayed, I'm sure you're dealing with a case of Ovarian Remnant Syndrome. This results when a piece of ovarian tissue is inadvertently left behind during the spay procedure. It can happen no matter what vet performs the procedure, no matter which technique they use and no matter which tools, although some vets feel that using a small incision and a spay hook, as they use in most feral clinics, is more likely to result in missed ovarian tissue.

Even if the vet sees they have removed the ovaries, if a small piece of either ovary is left in tact, and the vet doesn't realize it, it can produce enough hormones to cause heat cycles. The cat cannot become pregnant (her cervix is sutured off), but she will continue to have heat cycles until the remaining ovarian tissue is removed. You should discuss the best way to approach this with the vet. Unfortunately, it's beyond the scope of what most low-cost clinics will do. But many vets offer discounts for rescue cases.

Good luck!
Jessica