I have a small pink-red dry patch of skin that is not itchy or painful on my lower buttock, is it a rash? what could be the cause?

It depends.: You might have been born with it. This happens sometimes, and is usually called a port wine stain, or simply a discoloration of the skin due to lack of pigment. It also might be a lesion/cut/bite that healed long ago and stayed red. If you have others, it could be a skin condition from some disease. It would be wise to see your doctor to be sure, or just be aware of it if you are ok otherwise.