Ovulate a week early on Clomid?

This is my first month taking Clomid 3-7 (TTC # 1 for 10 cycles). I do ovulate on my own, but we have fertility issues through my husband and have had 1 MC. Anyway, I was told to start OPK's on cd 10 and expected to ovulate around cd 16. My first opk was really dark, basically a positive, then they consistently have gotten lighter and today cd13 a second line is non-existent. So I think I ovulated cd9! Has anyone taken Clomid and ovulated that early? I'm going to continue testing to see if I get another spike around cd16-18. Luckily we just happened to BD cd 7&8 so we're not out if I did!!