Hello KittCat,
Well first, I think (but what do I know) that if this persists, the best course of action is to go to your doctors and have tests done. Many different things can account for feelings of sadness and being tired. For example, at one point I was really sad and tired all the time and it turns out it wasn't due to depression at all but a very easy to fix medical reason.It was so simple to fix.And if you are depressed there are things that can be done to help you :)
I would like to say tho that I have had periods like that where I was extremely tired and sad for a few days.It passed. Sometimes we just have a few really horrible days. So hang in there and remember this too shall pass.
In the meantime be good and patient with yourself and try to do little things that will make you feel better :)
-Diva