My mommy cat is 4 years old and this is her only litter of babies, She had 7 by c-section, we have 3 left. The first 10 hrs home she would not nurse but since became a great mommy, ( we bottle feed and stimulated them)While checking on them today I noticed that one of the babies tail has died and is going to fall off... what could cause this?

You need to take mommy and the babies to the vet. Now.