Don't worry about it it is just because of lack of exercise muscles become weak and pain starts .you should start with strengthening exercises of quadriceps and hamstring muscles on sitting on high chair and lifting both legs up while keeping both feet upwards and then down.half squatting , and start 10 to 15 minutes walk also
Next Steps
use hot pack for pain relief and then start exercises
Health Tips
avoid cross leg sitting and sitting on low stools use comfortable footwear