After working with my personal trainer this morning, (we were doing cardio on a treadmill first to get my heart rate above 140; I am 57 years old, weigh 157 lbs, 6 ft tall) when after doing 3 sets of treadmill then weight lifting, I had trouble breathing. I told him it felt like my throat was closing up and I was gasping for air. Is this something to be concerned about? (The trainer had the treadmill at a 15% incline, 3 MPH, and had me go .15 miles...)