Beth, age three, has yelped a few times while lying down in the past week too, which she never did before . . . no moving or anything after, and she's full of energy otherwise. At least one time it looked like she had been sort of lying with her foot tucked under her chest. She hasn't done it for the past few days that I recall -- since it cooled down a bit, now that I think of it. This isn't any help to you, but I'm interested in tracking what you learn!