Hm, little confused. You had your last period after you and your boyfriend were not together? Is there someone else in the picture that could be the father?
If your last period is August 17th you conceived sometime between August 31st and September 7th. Sperm can also stay alive in your body for up to 5 days. So if you had sex August 26th- August 31st that could also be the father. Who ever you were having sex with during the the time from August 26th- September 7th is the father. If it was more than one person the only way to know for sure is a dna test.