I Have Generally Kept In Good Shape, Playing Sports And

I have generally kept in good shape, playing sports and working out. Over the past 18 months, I have stopped. Over the past 12 months, I get tired and out of breath real easy even after just minor exercise, such as walking up the stairs. I am 47 years old, 6'00" and weigh 175. Could I just be suddenly out of shape or is there something else to worry about?