My eyes have become a different color over the last year? ?

I was born with dark brown eyes, literally. Intact, my eyes were so dark that you could not see my pupil in the sunlight nor under magnification. Most people thought that my eyes were just very dialated. I had dark brown eyes up until I was 15. This past coming year I noticed that my eyes were becoming lighter, as you could see my limbal ring, a direct cause of my dark eyes gradually losing depth into a misty light brown, then hazey, honey, and now a vibrant green. What is the scientific cause for my rapid change of color? I have seen my optometrist and she has only given me reurance pertaining to the safety of this change. I know melanin and many other things contribute to the color of ones eyes but . . ok whatever. thankyou!