I took in a stray, young (under 12 months) pregnant Manx (Rumpy) cat a month ago. She delivered 2 healthy kittens 3 days ago (1 Rumpy & 1 long-tailed). She was too young to have any maternal instincts and helped very little in delivery but after a bit did begin nursing. After 2 days all except 2 of her teets dried up, however kittens are still nursing regularly and seem healthy. One (1) day ago she stopped using litter box. It is very hot here now and she frequently lays on tile floor away from kittens when they're sleeping. She is eating canned premium kitten food as often as she wants throughout the day, but seems to be thin. I have not seen her actually drink any water which is kept fresh next to her wet food and dry kitten food (she will not eat dry food). Is she constipated or dehydrated? If so, is there something I can do at home? She has been to vet 3 times since I took her in, but this was not a problem then.

You really need to talk to a vet. It's possible that she's dehydrated. It's also possible that it's something else. There's really no way to tell without having a vet run tests.