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Are they around the anal opening area or above on the hip area or at the base of the tail?
Does your cat go out?
Ok I'm guessing from that that they are above the tail not below it.
This could be an infection due to an abscess from a puncture wound. If your cat goes out or might fight with another animal in the house just a small puncture with one claw or a tooth can leave infection causing bacteria under the skin. The hole closes up and is nearly invisible and can heal while the infection is growing under the skin.
You can read about abscesses here
This could also be from flea bite allergies causing skin infection.
As you are seeing what sounds like an infected drainage you should see a vet but you can try some home care until then.
For home care you can use a warm/hot damp compress on the area to help it stay open and drain of infection. This will also stimulate blood circulation to help fight infection. You can trim the fur back away from the wound area to help keep bacteria out of the sores. You can clean the cuts out with Betadine iodine solution, chlorhexiderm solution, sterile saline, or 3% hydrogen peroxide. Then apply Neosporin or Triple Antibiotic ointment to the wound or any scabbed over areas. Keep the cat from licking the ointment off. It won't hurt the cat if it gets licked but it does better on the wound than on the tongue. A bit of hairball remedy on the roof of the mouth can keep the tongue busy while ointment soaks in.I'd do this 2-4 times a day and if you see no improvement or the infection seems to spread get to a vet for oral antibiotics (which would be a good choice anyhow with an infected injury). Hope this helps you!