Cats can be prone to cystitis which is a condition where the cat wil strain to urinate and have difficulty passing water, sometimes passing only a few drops of blood which is caused by straining. Potentially cystitis is a serious condition because the cat needs to get rid of the fluid which will be building up in its body. It is therefore important to seek veterinary attention as soon as you suspect that your cat is suffering with this condition.
There may be two main causes of cystitis, one is due to eating dried food and not drinking enough water. If you can, syringe water into your cat's mouth at the first sign of cystitis (or at least encourage it to drink water) prior to visiting the vet.
The next main cause is stress. Stress could be anything from visitors to the house, a new baby or even just a new cat next door or tensions in the household. If you suspect this is the cause, try to reassure your cat and again try to get it to drink more fluids on a regular basis.

 It is stress related, and can especially affect cats who live with other cats. It is recommended by "experts" that she should be a solitary cat. However, this isn't easy if you have other cats.

Believe it or not, my vet has recommended cranberry juice - if she won't drink it from the bowl, dip your finger in and let her lick it off. I promise, it works.

 My cat suffered from the same condition. She had repeated attacks of cystitis. She went through pain pills, antibiotics, anti-anxiety etc. The solution was to only feed her wet food. We started feeding her Wellness Brand Wet Food only and problem magically resolved. Try it.