How do you keep the Lower Back and Upper Glutteous muscles warm in winter?

I'm wearing bike shorts and tights, but with sweating, these muscles get chilled and then they cramp up on me.

It's not too cold here, in the 40s thus far.

IDEAS?

Winter-weight bib-shorts or bib-tights, and a decent length base layer over the top: I wear Endura 3/4 bibs and an X-Wear technical base layer under my windbreaker. Toasty down to -5 (25) or so.