A 2* seeker can do 240 pierce and/or 200 blunt. Both of those are way more than the block armor of the shield. All of the excess damage get's applied to your stagger limit, which, at 190 health is 76. If that is exceeded, the block fails and all the damage gets through.

Figuring out exactly how much the character actually takes at that point is complex, but I think that is what happened. If it got in a second attack while you were staggered, it almost certainly would have finished you off, but I think it's possible that just one could be enough.

20/20 hindsite alert: You really want to have bonemass power when fighting a 2* seeker! :-)

I don't know what the rest of your armor is, but this situation is also why I don't think it's obvious that the root chest piece is the best choice in mistlands. It helps against that 240 pierce attack, which is the worst, but you're more vulnerable to the 200 blunt attack than you would be if you have full padded or carapace.