You are not alone Lalobear. I too have worse symptoms on rainy days. Even before I had full blown AF I used to get headaches and feel less energetic. I think it does have something to do with vitamin D, but more so the change in atmospheric pressure that affects our bodies. And we with AF are already soooo sensitive.
I also attribute it to my candida coming out to play. The little b'stards just love a damp and soggy environment. And living in Florida I sometimes feel like a poor old salmon with one fin and half a tail trying to swim upstream when it comes to that issue.