My three show pain in different ways. One thing they all have in common when theyre in pain: theyre not active at all. I could have the vacuum going (they hate it) and if any of them are in pain, they wont move from their spot, even if the vacuum is inches from their pen (when normally, they would run to the opposite side of the pen).
Blue will sprawl and his eyes will glaze over. His ears will be flat against his back or out sideways. He wont answer to his name or any of his nicknames. If I pet him, he will wince. If I post the part that hurts, he will kick then move away and sprawl in the same position again. He will eat, which is great, but he will only eat what is near him. If food is too far, he wont get up to go get it.
Bombur gets aggressive when hes in pain. He will kick and box. Even if there is nothing there, he will grip onto something and grip and box. Often he will do so so hard that he will start to rip his own fur out from kicking so hard. If you try to pet him, especially in the place it hurts, he will ensure you bleed. He will not eat and he will not drink; he will need to be force fed.
Badger loafs. His eyes will glaze over, and hell keep them half open. He wont fully close them and he wont fully open then. Normally when you pet him, he will press against your hand and if you pull away, he will run back to your hand and shove his face into your palm; when hes in pain, he wont react at all to pets. He wont react if you call his name. He will only eat pellets and will refuse everything else (even water).
For your bunnys condition, has he been tested for UTI problems? My Bombur is prone to UTIs (and stasis, among other problems) if we stray from his diet. He needs a very simple diet of hay, water, and very few timothy pellets. He can only have a few greens (kale, spinach, and cilantro). If we stray from his diet, even to try a new veggie, it is very likely that he will get a (bloody) UTI. Maybe your bun is similar? Or has very acidic urine and needs more water based items in his diet to help dilute the acid?