My knee swells really bad

My knee swells really bad when I run. I had knee surgery about 1 1/2 months ago or so maybe even 2 months ago. I have been able to ride over 10 miles up and down hills no problem, regular walking no problem, even loading big rigs(my job) no problem. But yesterday I did my first 5k since the surgery it was about 10:30 am and at mile 2.5 or so my knee got really tight and by the time I got home is was pretty swollen. It stayed swollen the rest of the day and all threw work. It stayed swollen until about 4 am today. anyone have any idea what could be going on?