On holiday recently, in the heat and walking more than normally, my feet and ankles became really swollen. I wore flip-flops for a fortnight as none of the shoes or sandals I'd brought with me would fit.
A friend mentioned medication and I discovered the blood pressure tablet I take every day has swollen ankles and feet as a common side effect.
I was prescribed Amlodopine a few months ago as my blood pressure was " borderline" high. Seems strange that while I was much more active than normal my feet and ankles reacted. I'd imagine that sitting too I much might cause it, but not doing thousands of steps every day! (A holiday which was knackering rather than restful. Big mistake!)
Problem has vanished now I'm home, it's colder and I'm less active. It's certainly a very uncomfortable condition.