I'm sorry to hear about the pain behind your knee while running. This type of pain that occurs during activity and eases with rest could be related to several causes such as muscle strain, tendon issues, or irritation of the structures behind the knee like the hamstring tendons or the popliteal area.

Here are some steps you can take:

• Rest and Modify Activity: Avoid running or exercises that trigger the pain until it improves.
• Apply Ice: Use ice packs on the painful area for 15-20 minutes several times a day to reduce inflammation.
• Gentle Stretching and Strengthening: Once pain decreases, gentle stretching and strengthening exercises for the hamstrings and calf muscles may help.
• Proper Footwear: Ensure your running shoes provide adequate support and cushioning.
• Avoid Overtraining: Gradually increase your running intensity and duration to prevent overuse injuries.

If the pain persists, worsens, or is accompanied by swelling, instability, or difficulty walking, it is important to consult an orthopedic specialist or sports medicine doctor. They can perform a thorough evaluation, possibly including imaging, to determine the exact cause and recommend appropriate treatment.

Please seek professional medical advice to ensure proper diagnosis and care.To narrow down potential causes, we will need some more information.Do you have any past medical conditions?