You get pregnant when you ovulate
To get pregnant you need to ovulate and sperm has to be able to get to that egg. Once the egg was released from the ovary (ovulated) you have about a 24 hour window for the egg to be fertilized by a sperm. Generally a woman ovulates between day 11 and day 21 of their cycle. But a woman could ovulate at even time.
The length of a woman's period can also change from woman to woman from month to month.
So it is possible for you to ovulate and get pregnant the day after your period but it is not likely.