I go through feminine and masculine phases? what is this?

i am a 16 year old girl, and i always find myself going through masculine and feminine phases. when i go through a masculine phase, i want to wear more uni like stuff (polos, tee shirts, hoodies, etc), and when i go through a feminine phase, i want to look all pretty and girly. when i go through these phases, i also start to act and think more like a member of whatever gender phase i'm going through as well. my voice even seems to raise and lower when i go through these phases! it's all subconscious. i don't try to be more feminine or masculine, it just happens. i don't think this has to do with ual orientation, either. i know what i'm attracted to and i'm not just "experimenting." i think i have always been like this, and it's not just due to my age. what is this?