If you take Clomid on the third day of your period/cycle, (and if it works) ovulation should occur
on cycle day 14, and your period
14 days after that. Implantation should occur
10 days after ovulation, and after the embryo implants it begins to produce HCG which is what pregnancy tests detect--so the earliest that most pregnancy tests would be positive would be 10 days after ovulation or 4 days before the period is due.