Anytime you have sex you can get pregnant. If you had sex on cd 6 and you said that your af can arrive 1 week early on cd 21. If you get af after 21 days your O day would be cd 7 as there is always 14 days between O and AF. Which means, YES you can be PG. Good luck and I hope whatever happens turns out the way you want it to.
Try downloading Femta .com . they have an acurate ovulation anf fertility chart you can evaluate for 50 days without paying anything and it is totally free.