The white dragon does 355 damage with all of its attacks with 15% armor penetration and the headbutts are unblockable attacks.
Assuming you have around 600 armor with that setup that would hit you for about ~188 damage
Raeil is also correct, there was some bug with shield master… I investigated it a bit at some point but didn’t spot the exact cause in that short time, just some clues… might look at it further when I get a chance thou.
The jest of it is that if you have shield master, under certain conditions it’s possible that you get the 4x increased penalty (which should be applied to your stamina drain) also applied to the actual health damage you take, so instead of reducing the damage, it amplifies it. Most likely this is what’s triggering off the unblockable attacks of the dragon.