I would also back up your OPK's with doing BBT. Using something like Fertility Friend really helps when you are starting out, as you will chart your temps and a pattern will emerge over 2-3 months. When I was doing clomid I took my BBT every morning as well as used the OPKs so I could see when I was about to ovulate (opks) and your BBT chart will back that up in a few days after a positive opk by temps staying elevated.
there was a couple of times when i had what looked like positive opks but when my temps did not rise much and came back down i knew my body had not responded to that dose of clomid for that month. Or like bellyfever said its possible your body tried to do it, wasnt ready and now its going to happen.
Either way, good luck. But I highly recommend doing both BBT and OPK's