I'm not sure if this has been discussed before, but I thought I'd bring it up as it happened to me for the first time and I'd never heard of it happening before.
I was doing the Reavers bane with my wizard and had revived to Gianthold after a party wipe on the white dragon. I thought I'd sell to make room for more loot, so I take off my +6 CON belt and switch to my Coin belt. I instantly die. After scratching my head for a while I realized that the reduction of 6 CON had dropped my hit points by 46. Having revived with only 10 hps, that left me about 36 short.
I think that constitution increases should only raise the total amount of hps you can have, and not the amount that you do have. Maybe if you have full hps and get a CON bonus they might go up to max, but it sure shouldn't decrease the hit points you already have. It seems like the game treats those extra hit points like a barbarian's rage hit points, or aid, and I just don't see a constitution bonus acting like that.
I know that INT or CHA increases/decreases for a casters don’t affect their mps unless they are at full, so why is con affecting hit points like that?
ps I had the same thing happen later at the House J apothecary. Fortunately I only went to -8 hps and just lay prone on the counter for a while.