I had this happen to a sidewall of one of my tires before, not on the Lex. I got mine from a local dealer so I just took it back in and they replaced it for free. It's been a few years since so I can't remember what he said it was. The bump was about half the size of an egg.
its called an embelism, its pretty dangerous actually. What happens is the inside of your tire gets eaten away where the bubble is. When you unmount a tire you can actually see the rubber powder that has come off the tire. Get it changed dont risk your life or others.