New Sensation

I suppose it is a normal sensation but it is quite new to me, only discovered after my first born moved out of the house and three thousand miles away from my protection, care, guidance, and help. When he calls with an issue, which is not very often, I feel panicky almost. My chest tightens ever so slightly and my breathing becomes the tiniest bit more labored. I think it is good that he is so far away so he can work on solving his problems himself. He does just fine with it. So why am I having such a hard time? This is one of the few situations in my life that actually cause me stress. Why am I even thinking about this? And what can I do about it? Is it normal motherly instinct that is making me feel this way? Do I think he is going to fail, or fall, or give up? On every level that answer is NO but I cannot shake this damnable sensation. It feels like…deep down to my core like…helplessness. Why?