Pain In The Center Of Chest While Running, Walking, Jumping. Started After Doing Pullups. Due To Pulled Muscle?

I am 66, male, and in very good shape. Take no medication except baby aspirin. I work out at a gym every 3 days. Recently I have developed a pain in the center of my chest that only occurs if I walk fast, run, or jump up and down. The pain is immediate, not after any time of exertion. I have had this pain for about 2 weeks. The only thing that changed in my workout routine is that I started doing pullups, a set of 2. The other day I did a set of 3 by mistake (lost count). Could the pain be a pulled muscle or ligament? I can do almost everything else, and no pain, including swimming.