It doesn't sound like it. You had a period after the August 11/13 date, and you would have conceived on September 10 or thereabouts if you were 6 weeks pregnant (counted the medical way) on October 8.
Well, well,well,...... It is quite possible because studies show that mot women especially those with a regular cycle ovulate 14-16 days before their menstrual cycle and if my math is right u had sex with this other young man day 13 and/or 15 of your cycle which would of been your fertile period and sex with your boyfriend during your non-fertile period. So sorry to say sweetie you have some explaining to do. Double check with your Ob be sure to give him/her those dates and he/she should definitely be able to give u a more sound answer. Good luck Hun!!!!