What Causes Bluish Discoloration Of Lips While Performing Exercise?

I am 40 years old and in great health -- I exercise 5-6 days weekly, at an intensity of 7-9 on a scale of 1-10, and I have exercised as such since my teens. I eat pretty well -- lots of fresh fruits, veggies, whole grains, lean protein; although I do have a weakness for sweets and sometimes soda. For the past year, I have noticed that whenever I am doing short intense cardio intervals, or when I am, for example, in yoga with my head below my heart, my lips turn bluish. I am concerned that there could be something wrong with my heart. I have had a normal EKG in the past year, but I wonder if it might make more sense to have my heart examined during exercise -- perhaps something only is obvious when my heart is working hard, not when I am at more of a resting heart rate. 1. Should I be concerned? 2. What do you think about examination as mentioned?