Turned out, thumb was indeed broken (Bennett's fracture). And she put me in a cast. She said it will need to stay on for 6-8 weeks, which is a little longer than I thought. I begged for a splint instead, but she said that doesn't offer as much protection as a cast and thus can delay the healing process if there is any contact with, or movement of, the thumb.
I hate this thing. It comes up to almost my elbow and feels tight (and heavy). Plus, it is purple. Are you kidding me?! The only other choice was neon green. There need to be better options for adults....like white, black, gray, etc.