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.
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?
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.