I had no idea what was happening, and hadn't put two and two together with the plane ride the day before (4 hours). I wonder if it being 6 days after the Voyageur helped or hindered, maybe it made it worse because my legs were still getting over DOMSitude? Or maybe I didn't notice it? Who knows.
Now I know what to look for. I should have gone straight to the doctor when it happened, though, and I probably would have been in the hospital for a few days, which wouldn't have been fun. Luckily it resolved on its own! I was told by my uncle in no uncertain terms that if it happens again I am not to attempt hill repeats to see if I can work it out. It's like nothing I've ever felt before, and now I know why! Maybe.
yeah, generally good advice to not attempt hill repeats when your chest hurts.
I hope it never happens again!