When I Climb Stairs, I Become Dizzy And Lightheaded And

When I climb stairs, I become dizzy and lightheaded and have shortness of breath. Based on recent tests, I know that my heart is in great condition, lungs are normal, veins/arteries are clear. Resting heart rate is 60 bpm, blood pressure ranges from 112/70 to 120/80. My cholesterol is good, weight is normal for my height. I do cardio 5 times/wk, yoga 4x/wk, strength training 4x/wk. I sleep well, about 7 - 9 hrs/night. My diet is very healthy, I work with a nutritionist. I'm a 62 yr. old woman who does not take any prescription medicine. Do you have any ideas about the symptoms above? One doctor called it presyncope.