Cat having diarhea

Question
Do you know what could be the cause?  This female cat (with two kittens)is outside and I've noticed her bathing more than normal.  Yesterday she had diarhea all over the back of her.  

Answer
Kelly,
How old are the kittens?  Are you sure she had them all?  This may be remains of a kitten left inside her.  She needs veterinary care.  Please take her to a vet. If she is a stray, please call a humane shelter.  If she is nursing kittens, she needs all the strength she can get, and if it is diarrhea or whatever else, it is not healthy for a nursing mama cat.

God Bless,
Becky