She’s got a flat spot

What looks like muscle wastage on the left side of her tail. I had put it down to weight fluctuations but when I rode her today she was definitely not comfortable so we came back after five minutes.

Sound in trot and not sore to touch. She’s been funny about her front end for weeks and is now on lots of joint supplements and had lots of massage which seem to sort the front end out. So possibly she’s changed her way of going.