Hmm, I've tried a few things but I'm pretty darn sure it's breathing, not muscular. I can almost feel it in my throat, but not quite. Very hard to explain. You know that feeling when you're breathing REALLY cold air (-20 or colder)? My ride on Sunday was really hard but the problem didn't really show up until Tues night.

I'll just keep an eye on myself I guess. thanks for the input.