I had that problem when I lived in FL and then in NC. Biking absolutely made it worse even using sunscreen.

Turns out, that for me, it was caused by BC pills. Once I went off them (I got an IUD), the pigmentation eventually faded. Now, 5 years later, it's totally gone and I am totally synthetic hormone free.

Are you on BCP and if so, do you have any other alternatives? Oh, and FWIW, my derm prescribed crazy expensive creams to fade it and said that was pretty much my only option. He never even once made the connection to the BCP hormones. No one did until I moved to NC and got a better OB-GYN and he made the connection for me.