2y/o painful limp 10 days. x-ray showed healthy leg. when i extend his leg back&forth at knee, i feel a pop behind knee/thigh. does it sound sprained?

Many causes of limp: Toddlers can have limp for many reasons. Sometimes a hip joint effusion or fluid can cause pain from trauma or virus. Toddler fractures are not always apparent on radiograph or not recognized. Most common fractures are nondisplaced spiral fx of tibia, calcaneal fx, cuboid bone fx, and fx first metatarsal. If evaluation does not include foot these fxs can be missed. Muscle or soft tissue injury .