If the protection didn't fail when you had sex with your first then the ex would be the father but there is no 100% reliable form of birth control.
Timing wise it could be either.
You had sex with the ex on day 8 of your cycle and your first on day 11. You normally ovulate between days 11 and 16 of your cycle. Sperm can survive about 5 days, the egg about 48 hours.
You'll need a DNA test to be definitive but odds favor the ex (again, depends on the birth control).