This is most likely to be a bladder stone or urinary tract infection, but could be due to an intestinal blockage or issue with the reproductive system. In any case, get her to a vet ASAP. In the mean time, if you have a syringe, put her on a tarp or something plastic until she wees and try to grab a sample. This can help the vet to identify whether the issue is with the urine or something else. The vet will also want to know: 1) is she eating, drinking, passing faeces and urine? 2) does she squeak while going to the toilet? 3) is she lethargic, hunched over, or behaving unusually? 4) is the blood extremely dark or does it look like it's diluted with urine?