Dabi had got hit by a quirk from a new hero, this hero said to have the power to change someone's body. Everyone thought it was all fine, as Dabi only had some symptoms like cramps and a mild headache. It was only when you visited him the next day to check if he was okay, did you realised that was not the case. Yes, he still looked slightly unwell, and not fully changed yet.

and he certainly didn't look half pissed seeing the altered body he now possesses in place of what he's used to.