How can I feel sad when I have no reason to be sad?

I am a stay at home mom and I have a 2 1/2 year old and a 5 month old and in the past 2 months I have had a severe, over whelming sadness. It has taken over my life and it is making me miss out on so many great things that are going on around me. Even on days that should be the happiest I can't even fake a smile..