Low Sex Drive at 21??

I've been with my SO for almost 4 years. He was my first and only. We are in a very good place in our relationship, low stress, ect. But for the past year and a half my sex drive has gone downhill, like disappeared. Like the thought of sex does nothing for me, masterbation doesn't even interest me anymore. And I used to do it every single day! I don't ever want to fool around until he's inside me, THEN I want sex. I don't understand it. Can anybody please help?