Character Analysis
Lupin makes it through the whole "transporting Harry" ordeal physically intact, but he's growing really suspicious of everyone around them.
You see, he knows that the Order had to have been betrayed in order for the Death Eaters to show up when Harry was being moved, so he roughly interrogates Harry as a possible imposter when he gets to the meet up point (the Burrow):
LUPIN: What creature sat in the corner the first time Harry Potter visited my office in Hogwarts?
HARRY: Are you mad?
LUPIN: What creature?
HARRY: A Grindylow.
LUPIN: We have been betrayed. Voldemort knew you were being moved tonight. I had to make sure you weren't an imposter.
It seems a little unnecessarily aggressive, but we guess we understand: If you've lost a bunch of friends to Dark Wizards, and people like Snape turn spy and then turn back, it must be hard to trust that you're ever really safe.