Probably from over doing it on feet all day. I’d recommend compression hose. I’m 5 months out and if I am up and about even with movement for much of the day I’ll have swelling. I’m retired so I can avoid this on most days, however compression hose would likely be beneficial for you. Elevate when you can. Ice with pain. I still ice after 2 -3 mile hikes. I had bilateral replacements. Still doing my exercises as I will loose the max bend in my left knee if I don't. Also review with your doctor regarding the pain.
Hoping for the best,