Why do I still have some leg pain and swelling even after the clot is treated or gone?

Depends. Sometime it is treated but still partially there, sometimes there is scarring 'battlefield left after the war' and circulation just isnt the same, but it is good to be on the safe side and have doctor check if your not sure if clot is coming back.
Postphlebitic syndro. Even when the clot is gone, there may be lasting damage to the drainage from that leg. You may always have some swelling and discomfort. Put your compression stocking back on.