Bursitis is the most common cause of hip pain, especially so with repetitive motions/movements. Ice and nsaids (non-steroidal anti-inflamatory drugs: aspirin, ibuprofen, motrin, etc) work best. If I have a long ride coming up I'll take 800mg's of Motrin prior to the ride. After the ride I'll ice down any aches and pains and repeat the Motrin prior to bed.
You are right in that you are probably experiencing growing pains as your body acclimates to more and longer periods of strenous activity. Stretching and strengthening will help the affected area as well.
However...I ain't no Doctor. I am, however a nurse with a bad back. I hooked up with a physical therapist to learn how best to stretch and strengthen some of my weaker areas that cycling was irritating. I still get sore, but I don't experience the pain that I was putting up with and had become intolerable. Please take the above advise with a grain of salt, and possibly go see your doctor just to be safe.