It could be a sign of a female infection. I would make a trip to the vet.
it may due to dry skin. i have many bulldogs and i breed. try mixing aloe vera with water and rub through the dogs coat that will help with dry skin
it could also mean roundworms.. my puppy had that and would rub his bottom frequently on the grass and carpet
All types of dogs have glands on their behinds that do become dry at times ie the itching or rubbing across the floor or top of shoes, it would probably be best to talk to your vet about this problem he or ahe will have the beat treatment for the problem.
It could simply be the anal glands need expressed. The way a dog does this naturally is rubbing their behind off the ground. If this persists, have your vet or groomer express them. Ic could also be the tail, if your bully has a coiled tail, it can become itchy make sure to clean deep into the tail pocket.