My cat had kittens 3 days ago now she disappear?

my cat had 4 kittens 1 of them was born dead so we only have 3. she had them 3 days ago in front of our porch and since yesterday morning she disappear and we have not seen her since then. so my dog stella has been taking care of the kittens even tho stella never had puppies of her own but it seems she knows what she's doing since she is taking good care of them but the kittens are hungry and i been giving them baby formula milk Similac. and i notice the kittens have fleas ALOT OF THEM and i don't know what i can give them,. i mean stella has never had fleas and now i seen them on her as well since the kittens were born. Also do you guys know why Tomasa the mama cat disappear ?? i mean she's been our cat for more than a year and all of a sudden shes gone :(