First day of AF is CD1. Your activity on the 3rd would not have been timed accurately for conception (almost two weeks after your last egg "expired"). Based on an average 28-day cycle, you should have ovulated on the 18th or so, so any activity in the days before and after that date could have been fertilization dates. It's 7-10 days, on average, after fertilization that implantation occurs, and symptoms hit as your body produces hcg after implantation. It's too early for implantation to have occurred just yet (think 25th-28th), so you are having PMS symptoms as your body prepares for either AF or implantation. You can start testing as early as the 28th, but you'll have your most accurate test results starting on the 1st or 2nd.
Hope this helps!