I had unprotected sex and day after got periods. is there possibility to get pregnant ?

No, not at that time: A woman cannot get her normal regular period and be pregnant at the same time. However, she can get pregnant a week or two after her normal period. If she has some spotting or light bleeding that doesn't look like her normal regular period, she can check a urine pregnancy test about 3 weeks after sex (when conception might have happened) to see if she is pregnant.

Yes: There is still possibility of pregnancy. If you are within 72hrs of sex can try Plan B to prevent pregnancy. Otherwise can wait 2 weeks after sex and do home pregnancy test on first morning urine. If negative, repeat in 7 to 10 days. If both negative unlikely you are pregnant.