Pain in left lower back/side flank? Kidney problem? Diabetes?

I am a 16 year old male. I think i get enough excercise, I play on a baseball team and i'm on my school's golf team (we have to walk and carry our bags on the courses) and i try to go to the gym a couple times a week. However I am a little bit overweight with a BMI of 26. I do not feel the main symptoms of diabetes but i'm a little worried since I am overweight and my Grandpa had diabetes. For the last month I've had an on and off pain in my left flank area. It seems like i feel it for a few days then it goes away for a few days. It usually feels better after sleeping or taking advil. Sometimes it will feel better based on how my posture is (standing, sitting, lying on my side) and sometimes it will not feel better no matter what i do. Sometimes it will feel stronger and sometimes it will just linger a little. I'm worried of Diabetes or kidney problems or a UTI. Could it be plain old back pain or a stretched ligament or something? BTW no blood in urine or pain when i urinate.