I would have a talk with them to let them know that it's not their fault and that it is an illness that makes you cry. So that they don't start feeling guilty and thinking they did something wrong.

I did this with my daughter when she was small. She would see me cry and ask "is it one of those days?" I would say yes. It let her know that I was okay but dealing with something I couldn't help.

I hope you get to feeling better soon.