Five week old puppy: Is this just puppy clumsiness or a real problem?

She is not developmentally normal, I don't think. I do not know what could be responsible for this, but I would have her seen by the vet again, and perhaps ask for a referral to a veterinary neurologist. I have seen one pup that sounded sort of like yours (he also had some perceptual problems... would walk into gl doors and once walked through deck railing and fell... did not seem to realize he was walking into empty space)... this pup had encephalitis, possibly acquired from a distemper vaccination. There are sometimes puppies born that just simply aren't "right"... sometimes they can go on to lead normal lives, sometimes they cannot.