she is in no pain. She is jumping, climbing, eating, drinking. She is acting like there is nothing wrong. But the base of her tail looks swollen, and fat. At where her tail starts, going up till about 5cm (towards her head) so just basically the base of her tail. When i scratch it, she mows and her back legs bend in. But other than that, she is showing no other signs of discomfort at all.
Answer by BiPetual
Could another cat have bitten her there? Puncture wounds from bites can seal in bacteria and cause abscesses. Sometimes these open and drain on their own, but because of the risk of deeper infection, I would take her to the vet for a proper diagnosis. If it is an abscess, they will drain and flush it, and give you oral antibiotics for her.