SheFly, I know exactly what you mean. About 3 years ago, we went on a Memorial Day trip up to NH with a group. The first day, it was 50 degrees, drizzling, and normal New England cr*ppy weather. It was a really strenuous ride, with like 4,000 ft. of climbing in 35 miles.
The next day was 80 out and sunny. We did a totally flat 50 mile ride and both my husband and I had to stop by the side of the road and rest. We were hydrated, etc., it was just the drastic temperature change.
Jolt, yes, you have it worse than me this weekend!