opk clear blue

So I am on cd15 and I had a negative opk this morning. It positive at 3 o'clock this afternoon. I am using clear blue digital with the smiley face or empty circle. My question is does this mean I am ovulating right now? I read that from when you get a positive opk you should ovulate between 24/48 hrs from the results...! What is everyone experienced ? I can't have bd today as my husband will be back tomorrow. So I am afraid I will miss ovulations this month...