I posted a question about my dog walking in circles a few weeks ago. You may find the responses helpful. I think it was an ear infection which has now cleared up thanks to an epiotic cleaner and antibiotics.
My cat started to circle about and do other things out of character and it turned out to kidney failure, which meant the waste products in her blood were affecting her brain causing a type of dementia. I think a trip to the vet for a check-up is required ASAP. As apricot said, it might only be something like an ear infection, but best get it checked out.