So I Live In The Bahamas, However, I Go To

So I live in the Bahamas, however, I go to school in the US. I always get darker when I'm over in the US, and I'm not sure why. Whenever I come back over summer break away from college in the US, my complexion usually returns to its original color. I'm thinking that maybe it's the change in soap from Irish Spring (US) to Jergens (Bahamas). It could also be the water composition. Any response would be appreciated.