Nobody can answer that question for sure. We can guess, but guessing only goes so far. It's not likely, but it's still possible. Sperm can live for a week in perfect conditions and can be more than enough time to catch an egg.
So I had my period on August 16th through the 22nd and sex on the 26th (started birth control on the 21st) was I ovulating during the time we had sex?
I cant tell you a probability exactly.. There is a chance that you would be pregnant... There is some sperm in pre-*** and BC takes a at lest 7 days of perfect use to be active... and even then I would recommend another form of protection such as a condom for at least 14 days-30 days just to be on the safe side.