Not a bacterial infection. I'd bet dollars to donuts there's a piece of the thorn left inside the thumb. Tell your docs they won't be able to see it visually or with MRI, but they can likely pick it up with ultrasound. The inflamation is the body's reaction to the remaining thorn -- not a bacterial infection. As such antibiotics won't help.
In fact -- check out the article I've included in the link -- show it to your doctors. It describes an almsot identical case, with a surgical remedy and a full recovery in 2 weeks.
it's not a bacteria but a fungus... sporotrichosis