In my late teenage years I had problems with cycling and my hips. Doc said it was an overuse injury that is specific to young people, especially men. The growth plate at the top of your femur where it is jointed to the hip can easily fall to bacterial infection if you overuse it. It sounds weird, I know, but it makes sense. Once the growth plate seals (somewhere between 19 and 24) you'll never have the problem again.
He said the only thing I could do was advil and ice, and to take it a bit easy until it calmed down.
That being said, it could also be your aero position, certainly. Maybe take a picture of your posture on the bike somehow and post it here to see what people think.