I agree with cataholic who said "I would search for the reason why a change in behaviour and look to recreate a positive memory for him."

Is there some sound that is bothering him (one that you cant hear)? I would think that something is bothering him & that he is not hanging next to you just to bother you. He must be scared of something imo. In my case, my outside dog Annie will not move from my side when I'm walking IF she has heard gunshots. We live in the country & just went through deer season & she stayed scared & next to me when we were outside. At first I did not pay any attention to the gunshots in the distance ... but she heard them...and reacted accordingly.

I'm just suggesting that perhaps your dog is hearing something that scares her. Dogs are very smart & maybe she senses something. Kay in NC Tues Jan 11