I had sex 15 days after my period and my period lasts for 5 days, could I still get pregnant?

Pregnant? The length of your period does not matter. What does matter is when you had unprotected intercourse. We count the days from the First day of your last menstrual period, not the last. Ovulation typically (but not always) takes place 11-21 days after the FDLMP or 12-16 days before the next period. But cycles vary, a lot. You might become pregnant.
Unsure. Most women ovulate about 14 to 16 days after the last period starts. So is you had sex 15 days after your period started you could easily conceive. If you had sex 15 days after your period ended (ie. 20 days after the period stared) then you are unlikely to conceive. However, if your cycle length is about 34 days between each period starting then you would have been having sex at your fertile time.