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The vet could check to make sure she doesn't have any allergies which could be prompting overgrooming, I assume she doesn't have a flea problem. She could be overgrooming due to stress, have you tried a feliway defuser? Jackson Galaxy recommends play therapy to help deal with stress as well, and that could help if she is also overgrooming due to boredom.
http://adoptfriends4life.org/pdf/Whats-Play-Got-To-Do-With-It.pdf
Higgs has been over-grooming herself, this leads to her arse becoming inflamed, which means she can't clean herself as effectively as she needs to.
She therefore scoots round in circles on the carpet, drawing lines/arcs.
I took her to the vet who basically shrugged and said "dunno mate".
How do I fix this?