It is slang. "I'm up for basketball" means I am eager and willing to play B-ball. I'm up for pizza" - right now I am in the mood to eat pizza. the way the question is phrased, the person addressed is feeling down or sick. The person asking is asking whether this person is feeling well enough or motivated enough to join in.

feel up to in means like, if you feel like doing it/ think you can do it.

feel up to (do something) to feel healthy enough or rested enough to do something example : I do not feel up to going to the game. http://www.americanidioms.net/feel-up-to-(do-something)/

If you think it's good, hopeful, gonna be great, etc, you give it a shot.