If you have do not excruciating pain, then it's most likely from growing too much and body trying to adjust to the weight change. This is natural and can come and go. This is also sometimes most common early in the day. Plus, did your mom smoke around you? Because you could be going through a nicotine withdrawal as well.
There are lots of reasons, but generally it is because there is fluid inside the knee. It come from a sports injury.
Usually these conditions are self-limiting and a gentle circumferential wrap with an ACE bandage and some ibuprufin is the answer.
If it continues, or gets really worse, you will have to go to a physician.They will evaluate you for a torn miniscus, which is also a possibility. Give it two weeks. Don't be afraid to ask for a cane. Stay off the leg.
The possible treatment involves extracting fluid from the knee with a syringe. It looks yucky, but doesn't hurt much. When they withdraw the fluid the knee will feel instantly better. They will examine the fluid for blood.
Sometimes, when this procedure is performed the problem goes away completely.