As others said on here, we often have no idea of how much our adult children are juggling and trying to cope with. So, when they do not seem to care it can be that they are just so weighed down with their own problems, they feel that if they try to tell you they may break down .
Also, even adult children have a fear of their parents dying. The OP's daughter has lot her Dad this year already, can imagine that when Mum said she did not feel too well, or well enough to help out with child care, she must have experienced that serious knot of fear.
Glad it is sorting itself out a little more now - Do think as older people we can sometimes get a little selfish in wanting someone around to care particularly when we are already feeling quite low and depressed due to being unwell. Quite normal reaction that, but with those with young children having such a desperately difficult time at present, can be just the straw on the back of the camel!!!