Dont do anything. Wait a week, if there is no improvement DEFINATELY get it checked out. It will take a looong time to get an xray or mri scan so do not hesitate. Thinking it would go away, but it not doing so is a bad sign! That means that your injury is not healing, but being further damaged. Best of luck to you and your knee!
It could be from biking, or you may have too much fluid accumulated in your joint. You need to see your doctor, but at your age, whatever it is, they can fix it.
Rest it as much as you can &if it doesn't improve, see a Dr.