I just don't have the energy. I don't feel well.
That's the way it is. Don't question me, call me names or make me feel like a hermit loser. I'm not antisocial (I actually love being with my friends), I just simply don't have the energy. I'm tired, my joints hurt, my jaw hurts and I don't want to move!
In a related note, that's also exactly why I don't like to make plans. For those who I'm in touch with regularly, you'll often hear "we'll see how I feel." It's not a scapegoat. Sometimes I really do just feel.like.crap.
I love you.