My rabbit (which I've had for only about 6 days) started thumping! I made the mistake of letting him run around when I first got him, and now that I realized that I have to keep him in his exercise pen for a majority of the time until he's fully potty trained then I think it's what's made him start thumping. It started when I let him out (for about an hour) after keeping him in his exercise pen for almost a full day. When he went back in the pen, maybe an hour later, he thumped and it just spooked me. Is this normal? I know that thumping is not a happy sign. Should I let him out everytime I hear him thump or will that just condition him to thump more? What should I do?