Trippin up with AK

[Terrabon98]

100 NL 6-max, stacks all pretty close to 100. CO limps, SB completes, I raise to 4 dollars from BB with AKo...only SB calls. Flop comes 5 6 8 rainbow. SB check/calls my 5 dollar bet. Turn comes a King, and he checkraises my 10 dollar bet to 20...i call (?)...river comes another king and he pushes...is this easy? I had no idea what to do

[BigBiceps]

looks like he flopped a set to me. what else could he have when he:
1) called preflop raise
2) check call the flop
3) check raise the turn
4) push the river

could also have something like K8s - lots will go in via SB/BB with that sort of hand