Positive opk on CD 23!?!??

Thought I wasn't going to get a positive opk. Got my darkest one ( but not quite positive at CD 18, and I figured I was about to O because I was getting ewcm and had tender breasts. But I took an opk this morning, and its a very strong positive! Confused because my cycle is on average 28 days, so it doesn't make sense to o this late. Any thoughts?