What could cause this???

Ok, I'm more than a little concerned about Pixie. We changed litters the other day and I know she hates bc she pooped outside the litter box twice now (I'm buying different litter today). I know it's her, bc I've seen Milo use the new litter for both. He doesn't seem to mind. So I pretty much now it's got to be her. Well the thing is she did it Tuesday and then she did it again yesterday. I was cleaning it up last night and I noticed that there was a some blood there also. I'm calling the vet today, but I wanted to know if anyone knew what this could possibly be from. I've never seen her do it before, but then again she usually uses her box and buries it really well. Should I be really, really worried??
She seems to be fine otherwise, she's actually more active lately than she's been in the past couple months, showing more interest in her toys, etc. Any ideas?

If you're going to change litter it should be done gradually by mixing in a little of the new, and a little more each day. I suspect you're returning to the old litter which is fine.....give them a rest and then try a slow transition later after good habits are re-established.

A little blood in the stool could be from an irritated bowel (hard stool, grass in the diet, IBD) or from worms or a parasite. If the stool is hard you could try a laxative (like laxatone, but NOT a human laxative), your vet could also do a stool test for worms etc, and if neither helped, sometimes a vet will prescribe flagyl for a week, because among other things its an anti-inflamatory. But a small amount of blood in the stool is not an ermergency.

Thanks Stephenq. The vet said it didn't seem to be a big deal, but I should keep an eye on her, which I will. Yes, I am switching back to the old litter. Well the old type of litter, she doesn't seem to care what brand of scoopable litter it is as long as it's scoopable litter.