When do you cut a run short?

In the olden days, my answer would have involved arterial bleeding. This morning it was just because I felt "off" and the snowy obstacles made the run damn tedious. I cut my long run short on Sunday, too. What's the matter with me? I haven't slept well since last Wednesday, but that probably isn't it. Am I sick? Maybe. Or maybe I'm just not motivated. Blech.