Exercise could actually make it worse @Suwal - the possibility of chaffing with flesh rubbing flesh. I know that I need layers to stop my thighs chafing when (or if!) I exercise - I can't do exercise in shorts or I would be rubbed raw.
Maybe back off on the walks for a little while - to give the area a chance to recover & heal? Can you go swimming instead and just be very careful when wiping the area after swims &/or showers. It takes quite a while for the scar tissue to 'settle down' & nerve endings to become less painful - and along the scar 'ridge', it could well be aggravated by the exercise as it would be higher than the surrounding flesh. The under arm could be whacking it each time you use your arms for balance whilst walking.
Cornflour can be sprinkled on areas where chafing occurs - it is very similar to baby powder but without the scent! ;)
Al the best - I hope it settles down for you with rest - then you can work on a plan to prevent it happening again. xx