I'm not a reef expert by any stretch of the imagination, but most of my university schooling focused on multispecies physiology. If the aneurism (or whatever the growth happened to be) was connected to a major blood vessel, its rupture could very well lead to the fish bleeding to death just as if it were stabbed in the same place. Given that it was so close to the gill tissue, which has a TON of blood flowing through it, there's a decent chance it was associated with a major blood vessel.
As for behaviour, if the growth wasn't pressing against anything, it wouldn't be causing much (if any) pain. The change in size could be related to a lot of things, but one option that comes to mind is a recent change in activity; racing around for a bit will increase blood flow to the gills, and that could force more blood into the growth.
If you still have the fish, try examining the growth under a bright light to see if there is a tear in it. If it burst from pressure, it will probably be a tear rather than a hole, like a balloon that's been stabbed with a pin.
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45g + 30g sump. 250w 14000K MH; remora skimmer with a maxijet 1200; gravity-driven overflow; fluval sea and koralia powerheads; oversized biowheel and 5 gallon trickle biofilter, both free! Snails, hermits, at least one little hitchhiker starfish, one dalmation mollie; pair of black ocellaris clowns, courtesy of this place!; then some sort of herbivore(s) and/or colourful things.
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