ovulation changes 8 hours later

I took an OPK 12pm today and it appeared to be positive. I took another just now, almost 8pm and it looks negative. What does this mean? I know a positive means you are ovulating 12-36 hours out, but since the one from 8pm shows negative does it still mean I can conceive possibly tonight ? I believe my results from yesterday looked positive. Thanks for your help! This was my first month using these so it's pretty confusing!!