Extreme pain my left knee cap?

Im 17. Stand at 5'11". Weigh 275lbs. I go into the weight room and do anything involving my knee and my left knee hurts. To be exact, it's my left knee cap off to left. It gets this intense pain. I do leg press at 480lbs 24 times a 3 sets. It's not difficult at all, it's easy. But I can run a mile as well and my leg will hurt. It doesn't hurt to stand up, it hurts when I bend my knee past 30 degrees and back. The farther I go, the more it hurts. I just got done atving in Oregon sand dunes and I thought it was getting out of hand. Anyone know what's going on? The pain was dull at first, the first day but now it's a sharp intense pain that last. Also, I know I'm overweight but im a football linemen so Im a good mix of muscle and fat. I could lose weight but my weight wouldn't effect my leg when I am riding an atv. Anything anyone?