Sorry, have only just seen this. I hope she’s either got better on her own by now, or you’ve been able to sort her out.
Sounds like cystitis? Maybe she’s got an infection from the mucky bum? When you get your hands on her, I’d check her temperature. I imagine cystitis is as painful and annoying in sheep as it is in women...
If she was in lamb, I’d think she maybe had a bit of a prolapse that was blocking the urethra, but it would be unusual to have anything like that pre-tupping.