Why do my eyes change colors?

My eyes change upon mood, light frosty blue for if im around people i like, and they turn to green when I am alone, and its been happening for 6 months, all my friends see it, and it starting to creep me out. When i am around my girlfriend, they are blue, but around my friends they are like a dark green, I have noted 4 different colors, Frosty Blue, Dark Blue, Green and Dark green. No one else in my family has this happen to them.