Take her to the vet. Wake up the kids and you all can get her checked out.

 

Also, bring the poop with you so the vet can see it.

 

Yeah that's serious, could be cancer unfortunately, yet i'm sure the cat's pregnancy plays a role so don't take my word for it.

Also, the black in the poop is also concerning. This one particular wet food i give my cats makes their poop black after it has dried out completely, but i determined that there are no other symptoms. However, black and blood in feces for people and animals in general indicates cancer.

I also remember reading that women sometimes have black stool during pregnancy. I see this post is a little old, did you get any information from your vet?