I am not a doctor and am not trying to diagnose, but you might want to google interstitial cystitis and see what you think.
This is the only bladder condition that even sounds like a rash, and according to everything that I have read so far, it is very hard to diagnose, and goes misdiagnosed sometimes.
Unlike a urinary tract infection, interstitial cystitis cannot be diagnosed with a simple urinalysis or urine culture. Rather, it’s a diagnosis of exclusion, which means that a clinician — usually a urologist or gynecologist — will first take a thorough history and then perform tests designed to rule out other conditions. These include infection, bladder stones, bladder cancer, kidney disease, multiple sclerosis, endometriosis, and sexually transmitted diseases.
The next step is a procedure called cystoscopy with hydrodistension, which is performed under general anesthesia. The clinician inserts a fiber-optic tube through the urethra and into the bladder. The bladder is then filled beyond its usual capacity with liquid or gas to stretch it and allow a closer view of the bladder lining.
The most common sign of interstitial cystitis is red pinpoint spots of blood (glomerulations) covering much of the bladder wall surface (see below). Sometimes there are scars or lesions called Hunner’s ulcers, accompanied by low bladder capacity due to tissue stiffening (fibros...
Here is the site with pictures...hope this helps. It is all that I can come up with considering the information that the doctor gave. Once again, Good luck.