Ok can someone explain this to me..Bella's ears stood up a few weeks ago. and today one of them is back down???

what happened??? poor thing looks so funny..lol. just so you know, she is 15 weeks old. ( in case that might matter) Riley has stand up ears as well, and once his went up, they stayed up. so I have never seen one go back down before. could something have happened to her ear or something?? or do they go down and then go back up sometimes??