Cat has had diarrhea for over 9 weeks

About 3 months ago, my cat fell from my upstairs and hit her leg on the staircase on the way down. She had a cut on her leg and was given an antibiotic shot. I noticed before her fall that she was occasionally having diarrhea, but I didn't think much of it as it was not happening often. After her fall, I began to worry a bit about the diarrhea and thought it could be related to her fall (even though there was some diarrhea issues before the fall). I took her to the vet for a stool sample, and she tested positive for giardia. They gave her panacur and metronidazole. Once she began on the metronidazole her diarrhea was much more frequent and happening every day. I took her back to the vet about a week after her medication ended and they tested again and said she tested negative for giardia. We waited about two more weeks, but the diarrhea continued so her vet decided to try a different food and gave me Science Diet Light (canned) since it has high fiber. I've been using the Science Diet Light for about 4 weeks, and there isn't much of a change.

I've contacted her vet again, and they suggested another fecal test, but this fecal test would cost around $250 as it is very extensive and would rule out any parasites. However, they did say other treatments that they would try next, such as steroid injection, could have worse side effects.

Any suggestions? I plan to go through with the test, and oddly enough, I hope it turns up positive so we can finally cure her. My concern is that it will be negative and we will have to do other treatments that can have bad side effects.

Thanks

It would be a good idea to do the test, as you mentioned it started before the antibiotics, but antibiotics can wreck havoc on the gut flora. I would suggest also trying a probiotic

Try an antibiotic, or adding pumpkin to her food.

You may want to try different foods to see if that'll help.

Nature's Variety really helped my cat and her runny stools! But there are a ton of different brands out there. I would look for a grain free formula. Just an idea/option

I would ask the vet for a sensitive tummy food to help her tummy calm down in addition to being tested for parasites and maybe even the more extensive PCR test for diarrhea panel.  My guy gets Royal Canin Gastrointestinal HE and it keeps the runs in check.  We mix it with a grain-free dry food and he also gets a grain-free canned food twice a day. 

How old is your cat? Antibiotics sometimes help diarrhea and sometimes they make it worse by throwing the gut further out of whack. I'm on my phone, so I can't write much, but perhaps this might be helpful:

http://www.thecatsite.com/t/283161/saccharomyces-boulardii-use-for-diarrhea-and-gi-disease-incl-ibd

I also second the suggestion to use probiotics.