The whirlies

I had them today. We went out to Looe Key Reef, snagged a mooring ball and were having a fine time. Somebody is feeding the fish there, that's for sure. We were greeted by big schools of yellowtail and sgt. majors. Every time we disturbed the water in any way, they came flocking over en masse. I am trying to think of a good analogy: like paparazzi on my heroine Paris Hilton? Continuing to have a fine time. Saw little yellow flying fish and a smallish Portugese man o'war. Deven went right in and saw barracuda and a shark (of undetermined ancestry, but BIG.)  And suddenly I am feeling a little queasy, had half a coke, felt better. Jumped in and paddled around, the water was gorgeous, perfect temperature, clear, clear clear. The various corals were stunning. Back on the boat and uh-oh, definitely queasy now and worse, feeling weak in the knees and hands shaky, so.... trip aborted. The whirlies however came when I staggered on to land and flung myself on the chaise. Bleh, I said in an extremely heartfelt manner. And it felt as if I was thinking with maybe the top inch of my brain and the rest of it was in standby mode. And now, 5 hours later, hooray I feel fine. Why does this happen sometimes and not others??