Those spots are most likely anal gland 'zits'.
Cats have a pair of small glands, one each side of the rectum, that may have a speck of the conrents showing at the opening - kinda like a 'butt blackhead'- it's normal.
The glands naturally empty with defecation or grooming, but sometimes they may become clogged. If they get blocked, you may see your cat trying to groom more excessivly or see a swelling in the area. If you see these signs happening, get the area examined by a vet,'express' the clogged gland and make sure it isn't infected.