Late ovulation

Hi, I just need some advice. I approximately have a 30 day cycle. This is my fourth month using OPKs. The first month I got a positive on cd 15, the second month on cd 15, the third month on cd 16 then this month I didn't get a positive until cd 21. Has this ever happened to anyone else? I'm just confused. Will be period be pushed back then? I'm usually pretty regular. The only thing that I did differently this month is had a pap on the 16th of December but I don't think that could be the reason for delayed ovulation. I'm just confused. Any insight would be appreciated. Thank you!