Blood in Litter Box (and I think my cat may be eating litter)

I have two female kittens, about three to four months apart in age.  One is nearly a year (10-11 months) and the other is about 7 months.  The older one is not spayed yet and the younger one is.  

This morning, I found a pile of damp litter outside the litterbox.  At first I thought one of them had just gone outside because I had forgotten to clean it yesterday and it was full, but that doesn't explain why there was a little mound of litter there, and it didn't smell like urine.  I can't think of an explanation other than that one of the kittens ate some litter and then threw it up...

Then when I cleaned the litter box out, I found some blood in the litter box.  There was not very much, but it was very bright red and it was in / on a stool, not with urine.  It was too much obscured by litter for me to gather more info than that.  We also found a large drop of blood on the bathroom floor about a week ago.  We assumed that the older kitten was getting ready to go into heat (though she has never bled during heat before).  

Right after I cleaned the box, the older, un-spayed kitten jumped in and defecated in it, and there was no blood, though she did have to seem a little trouble going at first.  

We have two litter boxes.  One is used by the older kitten almost  exclusively and the other one is used mostly by the younger but is sometimes shared (this is the one where I found the blood and the possible vomit).  Both kittens are acting and playing normally, as well as eating and drinking basically normally.  (Our older kitten has never liked to drink water, but she has been doing better the last few weeks).  As far as eating unusual things (other than litter), our older kitten really likes to eat plastic, but rarely gets it, and our younger will eat almost anything, though I haven't seen her get a hold of anything lately, and I do my best to keep small things put away and cleaned up.

There are so many variables here that I don't know what to think or do!

It might be possible that they, or one is constipated and hence the blood in the stool. As for eating litter, it's usually a sign that they might be missing some nutrients in their diet. Can I ask what they're eating?

They eat Purina indoor cat chow.  Every once in a while they get Purina whisker lickin's cat treats.  I used to supplement the dry food with Friskies wet food, but I stopped that a few weeks ago because I was doing it just while they were getting used to each other.

sometimes cats eat litter if they are anemic. and internal bleeding can cause anemia. usually if a cat has anemia, they may also have pale gums and low energy, among other symptoms.  

If kitty is eating litter, or you even suspect it, please take the cat to the vet for blood work. Young kittens may explore and try eating new things. But litter eating is so commonly an indication of anemia, I wouldn't chance it. Our Tuxedo - at first - also played and ate normally.

non-clumping is recommended for kittens, as others said a trip to the vet should be made for blood work

oh, that is scary!  since i don't know what kitten it is, should i try to make an appointment for both?  we don't have a ton of spare money, but i wouldn't want to wait and observe them to see who is eating it just to save money if it is serious enough that that could hurt one of them.  do you think it is okay to wait and see if i can figure out which one it is?

You can call the vet to schedule an appointment to bring them both in. Let them know what happened and that you just want to test them to see if either is anemic. That way if no one is, you haven't spent the money for full blood work before they're due for the annuals - and if one is, you can then pursue diagnostics and tests. The test for anemia is called running the "hematocrit," and they can do it in-house (or most can), and here in NJ it costs about $18. I don't know how much your vet will charge for the appointments though.