blood in litter box

Question
I have two cats, one female and one male, and as i was cleaning out there liter boxes today and i found some blood next to some fesces and it almost looked like a bubble in it.  My problem is i don't know exactly which one it is coming from.  

Although my female cat did have giardia about 6 months ago and we did treat both cats, the vet gave us antibiotics so they didn't keep passing it on.  It hasn't seemed to come back at all and I never saw blood in the litter when that happened.  I just want to know what it could possibly be. Thanks!

Answer
Sometimes cats will pass a drier stool than normal and it will irritate the inside of the rectum and fresh (frank) blood will then follow the stool out.
It is usually nothing to be alarmed about unless it happens a lot and then I would suspect the food.
Also older cats tend to get pockets in the colon which can lead to many problems like this. Again it's a dietary issue usually.

No worries unless you see it often.