Me. I had this same exact issue in my Roka Maverick every time (1st gen I raced in for 4 seasons). What suit are you in? I assumed the suit was pulling on muscles a bit that did't ordinarily get that lift without it, or maybe lengthening me a bit, however subtle. Who knows since I never figured it out. I never got quite to the point of feeling like spasms were coming, but it was sore without fail and then the first few miles of riding aero felt great to stretch it out.

Last season I switched over to Xterra Vengeance which was a suit that always intrigued me and haven't experienced it since. Not even once. And I love the suit. Could be your suit design on your body somehow. Probably not groundbreaking, but that was my experience.

Good luck.