YAY!!! Congratulations!!!
I've never used a OPK and didn't know what the heck one was until I just saw your question and googled it. Apparently you can use a OPK as a HPT and get it to tell you if you are pregnant. So it sounds to me that it could be detecting LH or hCG, either of those two hormones can be detected by the OPK because they are almost identical. If I were you I'd go out and buy a HPT now and pee on it...if you get the positive then you'll have your definate answer. If it's negative then you are ovulating very late in your cycle and you'll have to do the "baby dance" again tonight. Either way, tonight you be doing a dance...the "HAPPY DANCE" or the "Baby Dance."
Best wishes to you and yours!