I could barely lift my legs high enough for the last 2-3 steps of a standard flight and had to rest at the top (and sometimes sooner than that) before moving on. Other symptoms
were a little worse too, including the distance I could walk before resting.
At other times in my life, stair climbing has been fatiguing or caused those symptoms of increased heart rate and heavy breathing. It happened when I had bronchitis, was anemic or had periods of irregular heartbeat.