This is so hard. I have to deal with it in other areas.
I've found with my daughter, who is eleven, I give her a very simple and short answer. I do not dwell long on the negative side of things in order to not scare her and I give the info in simple words, as much as I can get the info she can handle to her but not too much.
"Mommy is okay but sometimes feels sick." Or, "Mommy is not well today. The medicine is to help and the naps are to help, too, so I can be better for you. I just have a headache and have to put my head down for a minute." And so on.
I have arthritis and sometimes similar things happen and I've told my daughter this. She accepted it and even asks, "do you feel ok now?"
I don't know if that helps any and am sorry for your hard time.
A person is a person, no matter how small. -Dr. Suess
Perserverance and spirit have done wonders in all ages.