Caffeine sensitive?

I think I may have reached a point in my life where I can't tolerate nearly as much caffeine as I used to be able to. I made a pot of coffee late this morning. It was dark roast, and so had less caffeine than a lighter roast like Breakfast Blend. I drank less than half of it, and not all at once. About 1:30/2:00 p.m. this afternoon I warmed up my still half full cup and downed a swig. Within a few minutes I felt twitchy, a touch short of breath, and like my heart was beating faster. These are all symptoms of caffeine overdose. I used to be able to drink as many as two pots a day, though that was a rare occurrence. I seem, though, to have been slowly decreasing my caffeine intake over the last year or so. Maybe I was subconsciously aware that my tolerance was decreasing, and cutting back in response. I know there were nights at work, toward the end, where I wouldn't even finish the thermos of coffee I brought with me every night.

Right now, an hour or so on from the start of my symptoms, they've dissipated. Strange that there is such a thin margin of tolerance.