Confused about ovulation kit?

hi i had my last period on the 10th march until the 16th march, i came off the pill on the 7th march and then got my period a few days later as u can see! anyway i am getting all the signs of ovulation yet when i do a opk it comes uo negative why?? could it be because ive vome off the pill and my menstral cycle might not of got back to normal yet so i might be using opk but at the wrong ime? any elp would be good thanks