i have read that it is best when done in the morning and while fasting. P4 tests are notorious for being wonky because your progesterone can spike and fall over the day. So dont be too worried if you dont get what you are expecting.
two questions? are you charting or know exactly when you're O'ing? many Dr. tell you CD21, but hopefully by how you phrased the question you know that 7DPO is not necessarily the same thing and that 7DPO is what is important.
secondly, are they testing to see if you O'd or if you have low progesterone? if the latter, do you have other signs that you do (besides the Chemical, very sorry for that)