Clomid on Day 3-7 and now prescribed to take it on Day 15. Why so soon?

My husband and I are trying to have a baby!! We are doing a second run of Clomid. This time I was prescribed to take 50mg of Clomid on Days 3-7, had an ultrasound on Day 15 (no ovulation / 8mm follicles) and prescribed to take 100mg of Clomid. I am to start it on Day 15 (today) and take it for the next five days. Why so soon?? The RN explained it to me but it still makes no sense. Should I be taking this when I'm on my next menstrual cycle?