This is a very common problem encountered in dogs and cats. The most likely diagnosis is a bladder stone. Rarely are urinary tract infections so severe that the owner can detect blood. It could be a UTI but either way your vet will need to see her. Expect a urine test and radiographs be recommended. Both of these conditions are quite painful so please seek veterinary attention immediately.
If there is any sign of blood, you should pay immediate attention. There are urine home-tests that you can buy, the PawCheck on Amazon or Petcheckup, they will confirm if it is a UTI, in which case you should either take her to the vet or give her holistic treatment.
I used the home test for Diabetes, it was very helpful. Result was negative, which was confirmed by my vet at the checkup visit.