Abdominal tightness during walking

I am 80 years old and basically in good health. I do have a minimal asthma situation and some heart issues being well managed by my cardiologist. I ride my bike approx. 150 minutes a week. When I walk more than a block, my abdominal muscles tighten so much that it forces me to bend my back and impedes my breathing. I have to stop for a minute or so, everything relaxes until I'm off to the next leg. I have been checked for hernias and stenosis with clean results. This started 3 years ago. I have no digestive problems.