what is this?

I woke up today and immediately felt the imminent feeling of doom.  I no longer felt the easy going, what-am-I-gonna-do-today feeling.  Where did this come from?  Why am I feeling it?
I can only conclude that as time lessens before I go to work (which I'm not dreading, or fretting) that this feeling could get worse.  It's affecting everything.  My decisions are no longer made with the clarity of a substantial mind.  Everything feels shuffled around like I can't get ahold of it.
I'm unsure of whether I need to go to the gym or take a nap.  There's too much going on this weekend to allow me to feel this way.  I don't really get where the feelings are coming from, because work is work - I'm not worried about it.  I am preparing for it, but at this point, it's a no brainer for me.  So nothing to sweat, nothing to worry about, no reason for the feelings of imminent doom.