Your sensory inputs remained connected at all times. That absence is unprecedented in its whole, but we do know that amputated members can hurt, and not just the stump.
Well I'm sure you doing the thought exercise is the same as killing yourself and then reviving yourself 4 hours later to see if you feel any pain, we should all really defer to your scientific ethical expertise in the future.
Just because your brain is where all feelings 'exist' doesn't mean you have the ability to manifest it at will. But the brain does plenty of things to us that we don't want. I don't think excruciating pain would be something a brain-in-a-jar would feel - but it's possible that being deprived of all sensory connections would cause things to get weird quickly. Sort of like phantom limb syndrome!
I've been hurt in dreams, in ways that do not appear to have been the real world poking through. It's never been great pain, but it's been real pain or something very close to it. There's also phantom limb syndrome [1]. It's not hard to imagine whatever mechanism is in play there could run even more unfettered when there is no feedback from the body at all.
We do not know that simply being severed from your body is simply experiencing no sensory input of any kind, and actually have a lot of reasons to expect that would not be the subjective experience for very long.
(Bizarrely, the dream pain has nothing to do with anything in real life that should have been pain. Apparently my subconscious is fine with dismemberment or stabbing or whatever, but what really bothers it is removing my orthodontic spacer from the roof of my mouth, which apparently it feels is me trying to remove the roof of my mouth. Subconsciouses are weird.)
Oh, good point about pain in dreams! I have also experienced this. I used to have a lot of lucid dreams, where I was often aware of being in a dream. I somehow hurt my left shoulder and was suddenly in excruciating pain. I woke up and my shoulder felt just the same as in the dream, very bad. I quickly examined myself for injury and finding nothing, the pain very quickly went away. I guess I cannot say for sure that it was not physical pain leaking into imagination land but that is certainly not how it felt.
Pain is registered by nerves connected to the brain, and given that phantom pain happens even when those nerves are truly gone, its definitely possible the pig brain could interpret it as pain. This is pretty new territory to make strong statements about
