While it is true that it is generally considered safe to have an intercourse right after your period, it cannot be used as a way to avoid pregnancy. If you have a short cycle naturally, it is possible that you could ovulate early, closer to the end of your period, thus increasing chances of getting pregnant.
Since ovulation cycles are typically not regular, it is possible that you could ovulate earlier than usual. Plus, a sperm can live for up to five days in a woman's body. This means that if you have an intercourse three days after your period which is very close to your fertile window and the sperm lives that long, you can get pregnant.
Ovulation is said to typically occur around day 12 to 14 of your menstrual cycle. However, there are many factors at play that affect your cycle such as lack of sleep, stress, depression, excessive exercising, dietary changes, and so on. Also, ovulation does not occur at the same time every month, so it becomes difficult to ascertain one's fertile days after period.