Stress Test--Failed

When I was in high school,my parents always planned family trips for the breaks immediately following the end of terms. Later, in college, I would travel cross country to come home for Thanksgiving or winter break, or spring break--right after midterms or finals.
And invariably, after late night studying, snacking, and post-exam celebrating I would be exhausted, and ultimately, sick.
My freshman year at Northwestern, especially, I remember coming home at Thanksgiving. (as the years went on, I decided that chaotic weekend was probably not worth the stress of an 1800 mile journey). I clearly remember laying on the floor, feverish, achy, and convinced of imminent death. My body never has coped well with stress, whether emotional, physical, or mental. I also have a tendency to push through events with unreasonable zeal only to literally collapse when events wind down.
Last week, there was not a single night where all four of us were home for dinner. I spent Tuesday in the car (from 7:20 AM to 6:15 PM), I wanted very much for E's birthday party to be perfect, S's school open house to be memorable, I had somehow tweaked my back and was unable to sleep at night and, and and...Until Tuesday morning, I woke up miserable. Well, waking up would probably be an exaggeration. My throat hurt, my head hurt, my body ached, and my eyelids insisted on drooping. Yesterday, I slept away S's entire school day. And went to bed at 9. Loser? Yes.
But what really strikes me is how much LESS stress I am able to cope with now than I was eight, ten or (gulp) fifteen years ago. In high school, I was taking 7 classes, running the school newspaper, working on college applications, and NEVER sleeping at night. In college, I was taking 3 or 4 courses, writing lengthy papers, and (ahem) socializing heavily. When I was married and kid less, I was working as much as seventy hours a week, running a franchise virtually alone, and traveling on the weekends.
And now? Now, I'm planning how many cookies to deliver to a 7 year old's party and I am beat, fried, frizzled.
What happened? Is it practice? Is the background stress of being a grown-up so intense that it goes on all the time and I don't even recognize it anymore? If that's true, can I stop being a grown up? NOW? Is it nap time yet?