I don't think it's a bug, that happened to Veronica too so I think it's because of the poison.
Not bug, she been poisoned. Thats why you need to carry alot of Antivenoms just in case you face any Cazadors.
Then how come I can stimpack myself and survive?
Or if my companion just gets hit once by a cazador they don't die from it?
I mean:
[Hypothetical]
Companion health 200
Cazador base damage 40
Poison damage 4dmg/5sec = 20 dmg total.
Companion get's hit once by cazador and I don't stimpack him/her and we kill the cazadors, total damage = 60.
Companion get's hit once by cazador and I stimpack him/her = Instant death.
If the game is meant to be designed like this, how come I don't die instantly from using a stimpack?
Either way it's a bug.
Either the companion should not die from a stimpack after being hit with a poison sting.
Or we, the players, should die if we use stimpack after being hit with a poison sting before we use anti-venom.