Hello and thank you for using HCM .I carefully read your question and understand your concern. But you should not worry,its nothing serious. As you describe, my opinion is that we are dealing with extrasystoles.This are electrical heart impulses that generates in some parts of heart different from normal sinus rhythm.This generate that escaped beat that you feel.You have this sensation like a beat is skipping,you may experience palpitations or an anxiety sensation. This extrasystoles may happen from different reasons from simple emotional stress to other pathologies like anemia,hyperthyroidism ect. In your case, my opinion is that walking is not the reasons of these extrasistoles. It is well known in cardiology that in ischemic heart disease the frequency of extrasistolic beats rises during exertion because of ischemia, but this is not your case. Because you don t experience chest pains and you don t feel palpitations wile walking but during afternoons. So,if I was your treating doctor I will recommend some examinations like a blood analysis to exclude anemia,the thyroid hormones,an electrocardiogram, echo cardiography and of course a holter monitoring.This will help us to document and see what happens with heart rhythm. At meantime I will recommend you to avoid coffee,caffeine products because they rise extrasistolic beats. I encourage you to have physical training because sometimes it helps in reducing extrasistoles,tachycardia, even making them disappear. This is nothing urgent or to worry abut but I will recommend the examinations if the symptoms interfere your normal healthy live. Wish I was helpful.Take good care.Best regards