Blood in urine- no UTI

My 9 year old female spayed cat Willa has blood in her pee. She has been peeing occasionally out of her box and there is a concentrated amount of blood in it. I took her to the vet after the first time, about 4 months ago. They tested her for a UTI and it came back negative. They said maybe it was just stress and prescribed and anti inflammatory and an antibiotic- Clavamox. The blood in her urine quickly went away.. then about a month after finishing her antibiotic it came back. So I went to a different vet. They ran blood tests and ultrasound of her bladder. Everything looked normal. So they again prescribed Clavamox. Now here we are a month or so after antibiotics and its back again? 

Blood in the urine is often a sign of bladder inflammation which is painful and causes the peeing out of the box. If the urinalysis showed no infection or crystals/stones, then the diagnosis is idiopathic cystitis which is just bladder inflammation of unknown cause. There are a couple of things you can do to help address this. Stress is often a cause in bladder inflammation, so investing in a Feliway pheromone diffuser which emits calm cat pheromones may help. Enriching the environment and reducing stressors also help. Cats like both high and low hiding places (think high shelves/cat tower and caves like boxes and cubes. Individual play time has been known to help. Removing or reducing stressors where possible (dogs, other cats, small children, loud noises, big changes in the environment) is also helpful.

Diet is something to address as well. If she is not already on an all wet meaty diet, then that change will probably be very beneficial to her. The moisture in the canned food keeps the system flushed out better than dry and meat is a natural urine acidifier. Removing all grains, starchy vegetables and fish from her diet will help as these can often by inflammation triggers. There is also a supplement (corn silk) which is known to be very helpful in reducing bladder inflammation in cats. It comes in powder form and can be added to her food. Most cats do not seem to mind the taste.

This is really helpful information- thank you so much! Do you have any suggestions for what type of food to switch her to?

This site (http://www.catinfo.org/) has a chart that breaks down the protein/fat/carbs of most commercial canned foods available in the US. You want a high animal protein/moderate fat/low to no carb food. Some that qualify are Friskies pates, Fancy Feast pates, Tiki Cat, Hound & Gatos, EVO, Nature's Variety Instinct, Addiction.