High altitude = good health?

I noticed something while I was working out this week. I like to use the elliptical machine for about thirty minutes when I first get to the gym (ok, that's really the only work out I do) and watch the TV. I almost always get there just in time for Martha Stewart, today she made the perfect chef salad with some lady. Anyways, the machines that I like to work out on have the metal squares set into the handles that can take your pulse and then show you your heart rate in real time. There's a little row of lights that start green, then yellow, then red. My heart rate is always displayed by the very last red light, blinking just as fast as it can. I'm pretty sure this means my heart rate is to high. This all changed last week. My heart rate has been in the green the last 5 times I've gone to the gym. I'm not talking about the top part of the green, close to the red, but middle green. This change took place the first time I worked out since returning from Mexico. I wonder if walking and working in the high altitude conditioned my body to run better? Is that possible in just 8 days? Anyone know more about this? Anyone had his happen to them? Anyone understand LOST last night?