See a doctor as soon as you can. Only they can tell you the problem and get you the treatment you need. In your case, see a knee specialist.
If all that is needed is rest and some PT, the academy should probably still be notified. I know of one candidate with a fracture that was set to heal before I-day, and dodmerb changed his status to incomplete pending an all clear from his doctor, which should be no trouble to get.
When I fractured my wrist, I saw a doctor and found out I would need surgery whose recovery would put me past I-day.
If worst comes to worst, (and I'm praying it doesn't), you would contact the cadet clinic at USMA, who would have you send paperwork from the physician's visit to DoDMERB. They would decide whether to change your status to disqualified, and then the academy could decide to give you a waiver and continue treatment while you're there (unlikely if the injury would cause you to fail to keep up).
Either way, don't freak out yet since you don't know what the problem is for sure. If your doctor doesn't think you would be ready for I-day, that is a reason to begin to contact the academy medical liaison.
Good luck to you, and I hope for a quick heal.