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.
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.