What Causes Skin Darkening?

I did not realized that my skin turned so dark, darker than summer...until I sat next to a friend who skin color was much darker than mine..then I noticed that we have the same skin color now. I ve been using the same face foundation for 3 years, don t use any body lotion, no diet change, nothing changed, and been having regular daily routine as usual. I been having some white spot (skin turned lighter) on my belly but that s not the case. what confuses me is that my skin turned so dark this fall. should I worry about it? Is it normal? People skin usually turned back to the original colors or turned a little lighter, but why mine s turning so dark? Thank you!