HELP!!! Gums are changing colors

Hi, my dog Salt has been having an issue for the past couple of months. I have taken her to 4 different veterinary clinics and none seem to find anything wrong with her. She's a 4 years old, 65 pound, white American Pit-bull Terrier. Usually high to mid energy and loves to play and be goofy.

Last night she was playing for a while and decided she wanted to go lie down. She NEVER gets tired of playing, but because it was way past her bed time I just thought she was tired. This morning when we woke up she was not her normal self and presented these symptoms:

White gums, trouble standing/walking, limping on right leg, hunched back, shaky throughout the body.

This clearly is a scare but this is the third time this has happened (I rushed her to the vet each time).

I rush her to the vet again and by the time we get there her gums are pink again and all symptoms are gone except for the limp. (this happened the first 3 times as well)

They run blood work again for the 3rd time and everything is normal. (keep in mind this is a different (4th) vet clinic) The doctor says, "Her blood work looks perfect and her physical exam doesn't show anything is wrong but she has a slight limp so we'll give her some painkiller. Just keep an eye on her and if it happens again, bring her back."

I have heard this 3 times prior and this has been going on for 4 months now....something is clearly wrong.

Has anyone heard of this before??? DESPERATE FOR ANSWERS