Question: Can I get pregnant if I had my period the day after I had sex?
Answer: It is highly unlikely that you got pregnant if your period came after you had unprotected sex. Though it is not entirely out of the realm of possibility.
A pregnancy test will tell you if you are pregnant or not. If you have an unexpected pregnancy test results, like a negative test when you are sure you're pregnant, wait a few days to retest.
If you are trying to avoid pregnancy, you should use at least one method of birth control every time you have sex. The various methods of birth control all have different attributes.