Thank you for your patience. There are a number of possible causes for your girl's apparent discomfort or irritation in her hind quarters. The obvious answer would be musculoskeletal pain (perhaps from jumping from a high or landing awkwardly), pain secondary to arthritis (more likely if she is an older girl), to an irritation from fleas (fleas tend to live by the tail base), or even impacted/full anal glands. Hopefully she is eating with no issue, and peeing and pooping fine. Keep a close eye on her toileting over the next few hours, as sometimes female cats with cystitis (an inflammation of the bladder) can also have symptoms like this. For now, make sure she has plenty of fresh water available and do encourage her to drink. You can offer her, her regular food now as well.
If she seems particularly irritated or painful right now, then you are best to make the trip to a local ER vet now if you can. Here a full physical exam, expression of her anal glands, and potentially (if necessary) an xray or two of her sacrum and hind legs, would help to find the cause of her apparent irritation. Hopefully she just settles in the next hour or two however so you can follow up with your regular vet tomorrow or Monday. If your girl isn't up to date with defleaing and deworming treatments, then please do this first thing tomorrow using reliable products like Advantage or Revolution for defleaing and Drontal or Milbemax for deworming.
Continue to keep a close eye on your girl tonight and hopefully this irritation settles pretty quickly. Let me know if you need any further advice here. I hope all of the above makes sense? If you have more questions or if I can help in any other way, please do not hesitate to ask! If you would like to accept my answer, please press RATE OUR CONVERSATION (I am not compensated in any other way). Bonuses are always welcome. Thanks! I hope to work with you again soon!
PS: If you have additional questions after you rate the question, you are welcome to request me for additional conversations if I am on-line or by beginning your question "Dr. E..." or "Pet-doc..." and others will leave the questions for me.