Boys are more likely to get blocked bladders (crystals fall into the Urethra and block it so the bladder just fills up and can't empty then toxins built up. it's a life threatening condition if not treated in a timely manner). It's rare for girls to have it due to their biology, although not impossible.
Could you not pop her to the vet today. Whatever it is (UTI/Constipation) she's probably feeling really uncomfortable- if it's a UTI the vet will give her a little jab which should make her feel a lot more comfortable. I would be concerned that there is always a chance she has blocked (if she's trying to go but isn't producing anything) and it really needs vet treatment asap as it is life threatening.
I'm not trying to scare you or anything, I just get a tad paranoid with bladder problems since Memphis' incident
She could also have the runs if she's constipated.
I would say vet today - just to be safe.