Is it possible to get pregnant from sex one day after a period ended?

Possible: If you had a short menstrual cycle it is possible. As an example- a woman with a 22 days long menstrual cycle would likely ovulate on day 8. That means her highest fertility would be days 7 to 9. The cycle length starts with day 1 on the first day of your period. As long as there are still viable sperm in the woman's body when an egg is available - then pregnancy could occur.