Yep, totally happy relaxed bun.
Putting the head down isn't really an indicator of pain. Usually when a rabbit is flopped down like that with the head down, it's relaxed. If it's laying like that and breathing heavily and rapidly, may be damp around the nose, and ears hot and flared, the rabbit is overheating. If a rabbit is sitting up with it's head down, say shoving it under your hand or laying it down while next to you, it may be requesting to get head rubs. If a rabbit has it's head and body pancaked to the ground with eyes wide, it's scared. So it's all about interpreting body language, posture, and other signs, to determine what it's body position means.
Signs of pain are usually being hunched up and not wanting to move much(different than when they are sitting sleeping), laying down and getting back up repeatedly shifting position, belly pressing(not usually with the head down) accompanied with other signs of pain, loud tooth grinding, squinting eyes, trembling, hanging head or paws in water dish, lethargy, lack of appetite. You are basically looking for behavior that is out of the ordinary for your bun.
