I have a mare she stands with her tail tucked and switches back n forth on her hind end the vets have tested hay feed bloodwork it goes away in 72hrs she is eating drinking peein and pooing has anyone had this happen any input would be greatly appreciated thanks

Have the checked her sacrum?  SI?  Sore hocks will make them switch from leg to leg.  Also I would check for EPM

Couple things to check..... my mare will act similar when she gets greasy build up between her teets. She will also get a vaginal " bean" in the lower folds of her vagina. I have gotten almond and marble sized ones out of my mare and she much appreciated it. 

Any type of infection (yeast etc.)/worms or not clean under the tail/teets.