Good that you caught the seatpost had slipped - run a band of electrical tape around it at the top of the seattube so you notice it right away.
You may still have some inflammation in the area, with an increase of sinovial fluid as a response to the injury. The popping sound when you bend the knee is displacement of the fluid and gases in and around the joint.
If you're not experiencing pain all good - but no reason not to do gentle massage of the tissue in the area to help flush toxins.
Also, you mention riding in hills and with a trailer - consider riding one gear lower as well as remembering to downshift before coming to a stop. Starting up again in a lower gear reduces the sheer force on the knees.