12th. S1

D:Hey, bro. Jason invited us to a party.
A:Oh. You wanna go?
D:I'm down, you?
A:I'll pass. I'm feeling down today.
D:Awww, that's too bad. Hope you feel better.
A: Have a good time bro.

''I'm down'' means I'm willing to do something. And we can also say "I'm up for that."
-Hey! Do you wanna go watch a movie?
-I'm down!

''I'm feeling down'' on the other hand, means I'm in a very very very bad mood.