Amitriptyline can be very sedating, at first. They gradually become adjusted to the medication and will act more normal over time. It is a tricyclic antidepressant and therefore takes a couple of weeks to become completely effective for inappropriate urination.
Amitriptyline does not usually cause a fever or other symptoms you have described. Expressing the bladder is applying pressure to the bladder with the hand until the urine is squeezed out. It can cause some transient discomfort, and inflammation to the bladder so that she could feel as if she has to urinate when she doesn't have to, which can explain her going in and out of the litter box 20 times.
Though unlikely too much pressure on the bladder could cause a problem. I am concerned that she has a fever now. I would ask your veterinarian to do bladder X-Rays and a bladder ultrasound on your cat to find out definitively is the bladder is normal, is not damaged or has any stones or crystals. I would also recommend a CBC and Chemistry. Any abnormalities found should be addressed.
Additionally, urinary tract infections have multiple causes. Your cat could have interstitial nephritis, which would show normal urine when tested.
An excellent herbal preparation to help cats with urinary problems is corn silk extract (available at health food stores), please get a tincture made with glycerin because most cats hate the tinctures made with alcohol. She could safely be on corn silk daily as a preventative and part of the treatment.
She should also be on a grain-free, high protein, zero carbohydrate, and CANNED-ONLY diet for life.
Please let me know if this helps!