Hi, understanding from my nurse is that the window period for the egg to be fertilised i.e ovulation is about 12 hours only. Of cos, there are also some websites that says 12-24hrs.
For myself, after I took the ovulation jab, I also tested +ve on an OPK the next day. However, the +ve result is due to the injection so that is why my doc is very confident that I will obtain a +ve result when I told him I will check. I think you are still +ve on 18th bcos the hcg is still in your body.
Dont worry so much. If you listen to your doc and had intercourse during those fertile days, your chances will be very high.