What Causes Pain In Arm While Exercising?

I am 72 and excersise morning...100 pushups...15 deep knee bends and 45 minutes of brisk walking on the treadmill at 36 mph and 2 degrees elevation...lately i get a pain just to the right of my left armpit after the first 20 minutes...It doesn't go all the way into my chest and I have no shortness of breath...could it be something other than my heart...I experience no pain during the 100 pushups when my heartrate is at it's highest...