It's difficult to help you without knowing more about your pet (dog or cat).  Her breed, her age, any recent history of trauma, other health problems would all help to give a better idea of your situation.  If this is a dog, one of the most likely problems would be an irritation in the lower back--the lumbar area.  Other considerations would be impacted or infected anal glands, damage to the tail, or some distress in her abdomen.

NEED HELP PLS I DONT KNOW WHAT HAPPEND?

 ALL OF    SUDDEN YESTERDAY SHE WAS PANTING HARD ND I NOTICE HER TAIL WAS STAYING BEHINDE HER TAIL SHE TAKE2 MIN TO CIRCLE AROUND WHE  SHE TRYING TO LAY OR SIT I WONDER IF  SHE  HURT HER TAIL PLAYING OUT SIDE? BUT ON WHAT?

 OR SHE SLIP DOWN THE STAIR? IM SO WORRIED HELP PLS   IM SHYMIAMOR@YAHOO.COM