Puppy has started just standing still on walks

A couple of weeks ago, my eight-month-old puppy started just standing still and refusing to move on walks — before this she was always running and pulling or sniffing (or heeling nicely if I had very good treats). I thought at first it might be that she had an ear infection or something and was feeling wary so I took her to the vet and he said medically she was fine but warned me it could be the beginning of some sort of reactivity (though I think maybe he’d misunderstood and thought she was only doing it around other dogs vs everywhere). Is this something I should worry about? Or just a phase puppies go through? She’s acting normally otherwise but if it is a sign she might be developing some sort of reactivity or have some sort of issue with something in the environment I’d like to try to deal with it early!