Is burning powdercoating toxic?

Wow ... hope you're feeling better by now Mope (three hours since you posted). It might be a little late but try drinking a glass of milk and see if that takes the edge off. If the headache hasn't started yet take a couple aspirins -- because when it hits it's going to knock your socks off.

Powder coatings are essentially plastic: acrylic, polyester, etc., and breathing them in isn't going to do you any good as you've already found out.

There's really not much else I can help with from here except to boost up a BIGGG x2 on KrazyKuda's advice up there: NEVER WELD ON POWDER COATING!! Next time use your grinder first to clear a spot for the welds, or instruct your coater to mask / plug those areas you intend to weld on later.

I hope ya feel better soon. < hug >