I have heard of several people here that have had to have ports replaced due to various problems, including me but I have not hared that one yet.
My first port decided to pop through my skin while my wife and I were at Disneyland during a chemo break. Suffice to say it was not the happiest thing to have happen at the happiest place on earth. It got infected and had to be replaced as soon as I got back.
I guess I can see how the port may get loose from the vein but how does it get all the way down to your foot? Was it located on your thigh and not your chest? How could it get past your knee? (that just sounds painful) You might ask your doctor about a different placement