i just bought a westie and she has a pot belly. i would like to know if this a health issue?

The most likely cause would be worms, I would say, but an all over vet check would be appropriate if you have purchased her from a breeder that has not wormed their stock as other health issues may be there.

I agree with Isabel it sounds like worms.  Did the breeders give you details of when the dog was wormed?

I would get the pup to the vets tomorrow if it hasn't been wormed as they can cause all kind of problems.