You would have gotten pregnant November 7 through 20, according to the August 1 - 13 due date.
It doesn't make sense that the first guy would have gotten you pregnant, because you had a period well after having had sex with him. (You had sex with him no later than September 30, right?) A period doesn't come when you're pregnant, in fact that is often how women figure out they are pregnant.
Also, of course, if you were wondering when a baby would be due if it came from sex on September 30, it would be in June, not August.