Get her to a vet NOW! She probably is really old and has an urinary tract infection, and if thats true then your cat may lose a lot of weight and have less appetite. It probably also means that your cat has had the infection for a long time. Some other signs of urinary tract disease is increased thirst, more frequency of urination and straining to urinate.
It could also mean that your cat has mineral crystals in their urine (calcium oxalate or struvite),which can lead to stones, but for females and sometimes males not urethral blockage,these stones can cause bloody urine,and if blocking a ureter,loss of the kidneys "upstream" can occur. Fortunately ureteral stones are rare. You may need to get the cat surgery if this is true.
The most frustratingly common thing that causes blood in the urine is called "Interstital Crystitis". It has blood tinged urine,increased frequency of urination and straining to urinate. You can manage it by diet modifications and increasing canned foods for the source of hydration.