It’s an unexpected side-effect of having swelling somewhere else. It doesn’t matter if the swelling started right there in your knee or all the way up in your abdomen. Swelling in your feet can still happen regardless of the location of the original source of swelling.
But why does this happen?
In the Middle Ages, Europeans used to think that blood just sloshed loosely around the cavity of your body with a heart to loosely keep things moving (thankfully, other places in the world at the time had a much better idea of how human bodies worked). Oddly enough, insects actually do have open circulatory systems that basically work this way.
As crazy as this image sounds, it is actually kind of helpful when thinking about the lymphatic system. Lacking a heart and the closed circulation system that the blood gets to use, the lymphatic system relies on nearby muscle movement to keep bumping things along, and gravity can dramatically affect it, for better or worse. In fact, the role of gravity is so important that when it goes away, you immediately start having problems with fluid hanging around where it shouldn’t. In space, astronauts often experience “puffy face bird leg syndrome” which is exactly the way it sounds. Legs have big muscle groups that are used to pushing lymph against gravity, so they keep fluid out well, and faces don't perform large movements much and are used to having gravity to help drain things. Hence, the skinny chicken leg look paired with an uncomfortably swollen face. (As if the plethora of horrific ways to die in space wasn't enough to dissuade me.)
Here on earth, gravity is of course your body’s, native environment, but it can still find a way to kick you in the shins...or feet. If there is excess fluid hanging around anywhere in your body, it can slowly leak down and make your feet feel puffy and stiff, even though they were otherwise fine. Yup, this literally just happens because your feet are the lowest point in your body and the lymphatic system is a pretty open one, so the fluid leaks through your tissues until it settles in at the lowest point.
Also, you tend to move less when you are recovering, so the usual, everyday movement that would usually keep things pumping just isn’t happening. Remember, it's a system that relies entirely on outside movement to keep things going, and you are simply going to be a bit more sedentary when you are recovering from a major surgery or injury.
All in all, because of the way the lymphatic system works, it is totally normal for swelling to appear in your feet after a surgery or injury even what happened had nothing to do with your feet. So don't worry, it doesn't mean much and it isn't generally something you should worry about unless it begins to get dramatic enough to be more than just "uncomfortable."
These are some simple things you can do to help relieve the puffiness:
- Elevate your feet
- Put a cool compress on swollen areas
- Move your feet, try writing out the alphabet with your big toe
- Get some compression socks for when you need to be walking/standing a lot
- Keep properly hydrated (keeps lymph thin and flowy rather than thick and sludgy)
- Schedule a lymph drainage session to help the source of your swelling as well as your feet