Ovulation Tester... Got a question!!

Whitney
Question ladies.... 
I took a clear blue digital ovulation test at 9am and it said I was ovulating. So I told the hubby that we needed to have sex that day. And I thought that it was great that we actually had sex the day before! 
Anyway, I took the ovulation test at 9am and told the hubby when he gets home "let's do it". Just out of curiosity I took another ovulation test around noonish and the test came back negative. Do you think there was something wrong with the tester? Or is that regular?