Blood in the urine indicates a urinary tract infection, stones, or crystals in the urine. Your girl definitely needs to be on antibiotics to treat this condition. If it's not treated, she will probably start avoiding the litter box and urinating in places she shouldn't. I would definitely advise a trip into the vet so they can get your girl started on antibiotics as soon as possible.
Please let me know if I can help further.
Feline Healthcare
16 yrs health care mgmt & issues relating to cats, reproductive issues and multicat environments