Keep going out for walks.
It's not uncommon for puppies to do this. Going for a walk for the first time can be a bit scary for them, a big world out there they've never seen before. The more he/she gets out, the less this should happen. Just don't praise/pet/reward your dog for needing to be "under your feet".
If keeping your arm stretched out prevents him/her from doing that, keep doing it for now.