Is this expression with "feel up to" sometimes replaced with "not up to it"?
Do people usually simplify expressions such as "I don't feel up to going to party." and "I don't feel up to walking around" into "I'm not up to it." when they are asked?

Question

Is this expression with "feel up to" sometimes replaced with "not up to it"?
Do people usually simplify expressions such as "I don't feel up to going to party." and "I don't feel up to walking around"  into "I'm not up to it." when they are asked?

Answer

"I don't feel up to it" and "I'm not up to it" can both mean what you say i.e. I'm not in the mood to do something.  
They can also mean one doesn't feel qualified to do something though to make this meaning clearer, you might say "I don't feel I'm up to it".  Hope that helps, Michael