I had sex on a non fertile day but was fertile the next day could the sperm have fertilized a egg the day after sex?

Yes: The day of ovulation (when the egg is released and able to be fertilized) in the menstrual cycle is not timed exactly. Fertility is not timed as an on- or off- event. If you engaged in unprotected sex and not taking birth control, you are at risk for pregnancy. Viable sperm can live in the female's uterus/tubes for up to 5 days, so it is well within the timeframe you refer to.