Always a chance because sperm can live in you for several days.

I thought sperm died in you minutes after they dont fertilize an egg

No, they can live for 5-7 days inside u

Sperm can live in a woman's body for about five to seven days. And, as long as the semen is still wet, the sperm can survive outside the body. If the semen has dried, been wiped off, or washed off, there are no longer any living sperm.

What if you're on the pill?

The pill is not 100% effective. I am proof of that. Eleven weeks pregnant, was on bc pill for 9 yrs, depo for two and bc pill for one more. One dose of antibiotics and I am due thanksgiving day. Anything is possible. The chances however are slim that your body would be ovulating at the same time you are menstruating.

You can get pregnant at any time of the month.
Sperm can live in a woman's body for 5 to 7 days, but the average is three days, most common is two. But you still have to be very careful!
Every woman ovulates at a different time of her cycle.