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.
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?