If it is swollen it may very well be a bite or serious infection from the splinter. Is the area hot? Does the chick act lethargic and lie on its side?
Regards,
Leaf
I think pictures will be key here. I would isolate the chick, remove the splinter, and make sure it is eating and drinking. Is the "bubble" transparent or is the chick's side just swollen? It may be a spider bite or bee sting.
Regards,
Leaf