My rabbit seems nervous when relaxed is he just conflicted?

Sometimes when my rabbit is stretching out on the floor or sitting in my lap he starts breathing heavy and vibrating a little. It seems to me like this should be a sign that he's nervous, but he only seems to do it when he's in a pose that gives the impression he's feeling pretty relaxed. Am I just misinterpreting his body language, or is he just conflicted about relaxation, or does he have like breathing problem? Thanks.