I would guess (assuming your cat has hard stool) that it's either/or a combination of crystals in urine and worms.
I'd try deworming your cat, and making sure her liquid intake is sufficient. This means wet food, as cats don't drink as much as you think. Most of their liquid intake is through food.
A dehydrated cat will often develop the feline equivalent of kidney stones (crystals). Which, if your cat's stool is hard, would explain the blood (from straining and tearing).
Otherwise, a vet visit to get kitty checked out may be in order. While the above isn't serious in the short run, there can be serious damage if the problem has been about for a while.