Why The Periods Are Missing?

hello . i dint get my periods since about 3 months . i dont exactly remember because i have irregular periods. i had sex with my boy friend on 5th may. we used condom. but yet im scared of bring pregnant. i went to a doctor n told i dint get my periods. she gave my cyclonoam e n p. i took 6 tablets last week. still i dint get my periods what does this mean? what shall i do now?