Mom stopped cleaning 10 day old kittens

[Silvia - Foster mom]

My current foster has given birth to 4 healthy babies 10 days ago. She takes great care of them and they are growing like weed.
As of about 3 days ago it appears she stopped cleaning them. She still watches over them, feeds and lays with them, but she stopped cleaning up after them. What could have caused that?
The babies are quite dirty and I wash them now. If I wash them, will that change the their scent to the point she could reject them?

At about the same time I brought home a new born kitten, to see if the mother cat would accept it, cause it's own mother didn't. I put the little guy with the (at that time) 1-week-old kittens and the mom didn't seem to mind at all and took the little guy in. Sadly, the little guy was already too weak, didn't eat and didn't survive. Could this have something to do with why mom won't clean her own kittens?

 

[Sarthur2]

It might, but it may be that this mama cat is just tired. Continue to clean the babies for her, including their bottoms, and put in clean bedding. As long as she is nursing things should be okay.

Is mom still eating well herself? Does she seem lethargic or unwell?

 

[Silvia - Foster mom]

Yes, Mom is doing very well. Social, eating well and really other than that she is not cleaning them she is taking very well care of her babies.
Thank you for the feedback

 

[StefanZ]

Are you perhaps giving the kittens some extra food? If so, this could perhaps cause she wont take care of their wastes...

Its also possible she is mourning the little guy.