If I had sex the day after the last day of my period, can I still get pregnant?

Yes, there is always some chance of pregnancy any time you have sex. You're less likely to get pregnant at that point in your cycle, but it's not impossible. Same with if you use protection or not. There is always some chance as no form of birth control is 100% effective. But your chances of pregnancy are much less if you do use protection.

You're a teen, why are you having sex especially if you don't want to get pregnant?