A friend owns a rental house in Breckenridge, CO (house is at 11,000 ft overlooking the slopes). Her manager had a book that she left for guests including the dangers of altitude sickness. She ended up firing the manager (for other reasons) and gave me the book to revise. She wanted to altitude sickness taken out because "she didn't want to scare people." I tried to explain it to her and she didn't understand. By coincidence, her next renter ended up at the hospital and was on oxygen the rest of their stay (a week!) So you don't want to mess around with it! If you really thought it was altitude sickness you should go to the hospital to be sure.

Were you wearing a heart rate monitor? I once tried to run on the treadmill in Breck by using my HR monitor. I thought I would just take it slow. I couldn't even get up a jog without setting off my high-end alarm. I haven't had altitude sickness, but I have frequent headaches and sleep a lot when I'm there. I've also discovered it makes my digestive problems worse.

My sister-in-law moved from Missouri to Colorado. She is a runner and cyclist. She said it took about 6 months for her workouts to feel "normal" at altitude.