A day after period? No Pregnancy chance is very low...The supporting environment for pregnancy in the female uterus just finished degenerating before a day .So the uterus at the time you had intercourse might have gradually getting ready to support pregnancy(if possible) when ovulation(most fertile period) takes place which is likely 14 days after a female has completed her mensuration(if her mensuration cycle is 28 days). So I advise you to not have intercourse(if you are not ready to get pregnant) from 10th day to 15th day of mensuration cycle as the sperm have capacity to live in uterus for 2-3 days. Also try to use some Condom or ask your gf to intake hormonal tablets(only after consulting doctor)like saheli etc to prevent pregnancy. Luck will not be always your side!