My son Nathan is 2 years and 5 months and has been doing this for a while now. He will just come to me for cuddles and say "I'm very sad" or similarly phrased down emotions. When I try to talk to him about it, he cannot always articulate it - occasionally its when I moaned a bit to sharply about something, or when his older brother won't let him do something, but other times it's just out of the blue. and he does look so sad that you want to hug him and fix the problems in his heart.
But I can't, so we are working on naming emotions and expressing thoughts nicely.
I have noticed that he sometimes says sad when he could also mean upset, angry or any other negative emotion. So we focus a lot on those and how they make you feel - ie giving him scope and ability to express correctly what he is feeling.
not sure if it will help or not

I also managed to find a series of lovely emotion books for kids. Here is the Amazon link to the "when I am feeling sad" book in the range. they are all lovely and use descriptions that make sense to kids and help adults to teach about them in a positive way.
http://www.amazon.com/When-Feeling-Sad-Trace-Moroney/dp/0769644260