Oxygen rich?

So those of you who know me know that I have horrible cardio. It's further exacerbated when I'm at altitude - the usual shortness of breath, etc etc.

Well, this weekend was spent at 7k feet at Black Mtn. Climbing was awesome (but what would I expect from Black mtn?). The amazing part was that I could run around with no acclimation and no shortness of breath! Sweetness!

The weird this is that I'm back at sea level and I'm facing shortness of breath as I walk around my apartment. WTF?! Is it just too oxygen rich for me to breath properly? I'm curious to see how tomorrow's bike ride is going to go...