Having to test several times a day and getting different results

I have been using the Digital Ovulation Kit. Started on day 8, testing first thing in the morning. I have been getting negative readings until I tried in the afternoon on day 15 at the advice of a friend to try again later in the day, then I got my positive as this is supposed to be my ovulation day according to the calendar. This morning I tested again and received a negative at 7am and just tested again at 3:30pm and it was positive. What is going on if I had not tried testing twice daily I would have missed it. The packages are only to test once and if I have to test more then once it costs a fortune. I am feeling frustrated not knowing if I am indeed + or -...HELP!!!