What could cause a stress fracture on the upper tibia (right below the knee)?

Stress fracture. Stress fractures are often caused by over use or excessive weight bearing of the problem limb. Excessive jumping, running, heavy lifting and heavy work can all cause stress fractures. Excessive training in sports can be the culprit. Be sure you are living a healthy life style to prevent problems in general.
Many. Trauma(micro or macro), bone density problems, metabolic bone disease, running or repetitive low grade injury. Should be checked out.