1) You failed your save, which gave you the sensitive echo. That also appears to have come BEFORE the damage, so at the time it hit, you may have been given the vuln from the dragon.

2) Breath attacks are based on HP. The dragon had BARELY gotten hurt, so you're taking a nasty powerful one. I've seen the green dragon in the weald insta-kill more people than I care to count.

3) If you're wondering, after reading your orc fight as well, I'm pretty sure there's some reasons you were getting hit by that acid super-hard as well.