Here's what comes to my mind:

Your basal body temperature drops for about two weeks before, so you'll already be a little cooler, but then hormones, like estrogen, can relax your vascular system, which can lead to body heat being lost more readily through that system and your skin.

I'm definitely no expert, but can tell you from experience that a hot flash causes you to become flushed and quickly dissipate a lot of heat through your skin, after which, you've lost so much of your warmth that you'll be freezing--- at least for a little while, until the next one strikes.

So my guess would be that hormones are causing your body to effectively dissipate more of your body heat.