I used feel this way quite often. I just sitting, somewhat alone*, in my room when suddenly I feel really scared. Like I need to get out of the room, just run to anyone I know, just to know they where there. I found the this seemed to stem from bottled up wants to something anything instead of just sitting there alone. I recently started taking piano lessons, and whenever I get this feeling, I just get up and start practicing. The relief came soon after.  Do you have a hobby? something you like to do? Because for quite a while I didn't and I got that scary deep stomach feeling at least once a week. I think your just frustrated with a monotonous situation. It all gets bottled up until your body just tells you to get up, just run, until your legs give out. And this is experienced as fear.  I don't have much a basis for this theory but it's what works for me. Get out, do something. Live.