Sigh... I think I missed my O

I started using OPKs last friday and the first test was about 50% as dark as the control line... but I thought nothing of that since everything I've read says thats a negative and since I've never tested before I didn't know what my cycle looked like. Then this weekend I was in a wedding so had no chance to test. When I started testing again on Monday I have no 2nd line at all. So I probably ovulated over the weekend and I missed it. It sucks too because DH and I were talking a big game at the wedding about BDing but then we were so exhausted (well I was anyway after having to get up at 6am for hair and makeup).
Does anyone else use OPKs and get the 50% as dark result?