Limping & stomach problems

"Hi:
My one yr old shih-tzu - (long story)
She started to limp, out of no reason on 8/31, Tues night. So I bought her to the vet on Wed afternoon,but the vet didnt find anything wrong,& thought she might just have a muscle strain,asked us to wait it out. Thursday, she acted really sicked, looking like she tried to through out,but couldn't, her breathing became really fast,& she had sticky saliva. So, I was nervous, called the vet, he asked me to bring her in on Fri. Friday, got her in, had a X-ray, the Vet was suspicious that she might have unusual object, but couldn't tell until he did a GI surgery. GI surgery was performed on Saturday 9/4. My pup has been recovering very slowly according to the vet. The vet was suprised she acted so lethargic because of the tiny mushroom that stuck in her stomach; the vet was expecting her to recover quicker after the surgery, but she didn't.
We got her back on Tuesday 9/7, she started eat some chicken Tue & wed & thurs night, and in better spirit.Had a re-check with the vet, vet said she is okay. But, she stopped eating again Fri,sat,and Sun -only eating apple sauce.
So, I became nervous again. At the same time, her limping became obvious again, this time, is her back leg (previously was hint leg). She couldnt even hold herself while urinating.
I bought her back to the same clinic, but another vet for re-check. This time, the 2nd vet said she has poly-arthritis.
On Saturday (9/18),i will bring her back to take out stiches, the 2nd vet said might start her on steriod.
PLease give me advices, what should I do? Why GI problem + poly-arthritis happened At the Same time??
Her blood work is normal, (shouldn't poly-arthirtis has high WBC count? ) -her joint culture is -ve, swollen back leg.

Thank you so much for your help