Im currently on 8dpo Im confused on when I ovulated. My cycles are around 26-28 days but I did opk and I ovulated cd 18. According to the app my period dont start until the 19

But I been cramping for 3 days now on and off like if my period gonna come down any time soon. Could it be implantion cramping or period coming early but if it is coming early then why i ovulated late? im so confused!!