OPK and clomid help please

So I was reading about clomid and it can make you ovulate earlier. I was wondering has this happen to anyone before. Also can clomid make it harder to test for opk? This is my first time taking this and I'm getting cramps on my left side. Should I call tomorrow to the fertility doctor? I don't want to miss my opportunity. I'm suppose to go in on the 21st for an ultrasound but now wondering if I should be seen earlier. I also took an ovulation test and it is negative as of right now. Any advice would help thank you ladies.