Ovulation Day Jump?

Is it normal for your ovulation day to jump around? Last cycle I didn't get a positive opk until CD 15 and 16. I randomly took one today (CD 11) just because I have leftovers and it came out definitely positive. If it was a day or two's different, I wouldn't question, but 4 days seems odd to me and I don't know that I've ever had it happen before. This last cycle was weird in that my period was 4 days later and it was lighter than it normally is... I'm just wondering if something is wrong (and trying not to get my hopes up too much)?