Blip has had blocked anal glands, but there wasn't any outward sign apart from a slight build-up of old fur - which is what prompted me to take her to the vet. Worth getting that checked out though as blocked anal glands are painful and need to be expressed by the vet (which is also painful, but quick).
PS If it does turn out to be blocked anal glands, you should probably consider adding a little more fibre to Jess's diet.