That Unfathomable Check Gas Cap Light

After putting gas in the car, the light comes on immediately after. It will stay on constantly. Comes on every time I start the car and roll forward about 20 yards and again, stays on constantly. After 3 days, every time, it will come on and stay on for about 10 minutes then not come back on again till I get more gas. Then the cycle repeats the same way every time.

Fixed small vac leak I found doing the cigar smoke test a few weeks back. That repair never affected this light.

Thoughts?

That Unfathomable Check Gas Cap Light