What does "burning pain subsides but the damage is done" message mean?

what does the “the burning pain subsides but the damage is done” message mean? I don’t remember being hit with acid or fire. Is this because of some of the radiation mutations I have like radiogenic?

You got bitten by something with a damaging venom (centipedes, wolf and web spiders, for now). Though if you managed to overlook that I might need to up the damage…

that’s weird… I don’t remember encountering any spiders