Check your fade touched. That does sound like a glitch, not a terrible one but still I'd try different armor.
I have tried the version of The Vestment of the Dragon Hunter that I found normally but the same thing happened.
Sounds like a bug, but need more information to rule out some possibilities in gear. You sure that's the belt you have? Because a Superb Belt of the XXX Pact could have the effect you are describing.
Can you post screenshots of the stats of all your crafted gear?
Do you have a Sigil of any kind on your armor? If so, what kind?
Since you mentioned deepstalkers, I assume you are in the Deep Roads section of Trespasser. So too soon for Anchor Discharge to be damaging you. When you get hit for all but 1 point of your health, do you have barrier and guard at the same time? If so, even more likely it is a bug. It's like something is doing poison or elemental damage to you. What are your resistances to each of those, according to the character Attributes screen?
I haven't equipped any of the Pact of XXX gear since I don't like the abilities they have.
I'm playing on Xbox One so little hard to get screenshots so I'll write the stats here:
Crafted version of Vestments of the Dragon Hunter
Armor Rating - 310
Item level 26
+45% Magic Defense
+141 Maximum Health
No sigil
Crafted version of Staff of Corruption:
177 DPS
77 fire damage
+32 Damage vs living
+21% Armor Penetration
+24% Attack
+70% Critical Damage Bonus
Berserk: +30% Damage Bonus, +300% Damage From All Sources
Master Corrupting Rune
No sigils on any equipment.
I am at the deep roads section but I think it's been like this even earlier. It first happened during the first fight with Qunari Shock Trooper in the elven ruin but I thought it was just that it hit really hard. Wasn't until I noticed my party members could fight him with much difficulty that I noticed something was off. I usually have both guard and barrier at the same time since due to Horn of Valor. When I get reduced to 1 hp, I get a full barrier from Guardian Spirit. My electrical resistance is 20% but I noticed something really wrong with my other resistances:
Magic defense: -222% when usually 78%
Melee Defense: -274% when usually 25%
Ranged Defense: -251% when usually 45%
Not sure how this happened
I did turn on the Walk Softly Trial before going into combat for the first time through when it's unchecked now.