What is this?!!

I am getting this weird feeling these days, I am doing my normal things and suddenly I feel overwhelmed with sadness. I have nothing to be sad about thank God all is generally well. Is it postpartum depression kicking in a bit late, cos my brain was busy getting rid of the palsy, or is it depression due to extreme tiredness from caring for sick children, my children have been suffering from one form of cold or other for the last 2 months; and it is very tiring and emotionally tasking to deal with sick children. I don't know all I know is that it happens.