Buns do flop when they feel good. They also “bite their teeth” when they feel good and relaxed. It’s called a tooth purr. It can be quite loud at times, at other times it’s barely audible but can be felt if you put your fingers on the bun’s head. A rabbit can however bite its teeth when it’s in pain too – that’s often a choppier, more staccato sound. A bun in that much pain is practically never interested in food or treats.
Sick buns don’t tend to flop. A bunny with gas pain can press the belly against the floor like they are doing push-ups, but poorly buns mostly tend to position themselves in (an uncomfortably-looking) loaf. They also tend to try and hide away. This is a deep instinct, a poorly bun needs to hide away or a predator could spot its weakness and target it. They are also unwilling to move from their spot.
Could it be that she is feeling so good and relaxed now that she takes the (for a prey-animal like a bunny is) rather big risk of flopping out in your presence? By your description only I’d say this is what it’s about. It can be scary the first time you see a bunny flop, rabbits can’t get down onto their side in a controlled manner so they sort of collapse into the position. This is because of their anatomy and perfectly normal.
Eating, pooping, enjoying headrubs – great signs. It could be that she’s just a very happy bunny.