Facing A Delay In Periods

Iam married for 6 years have a 2.5 years old daughter always have regular periods have a 28 days cycle periods are there 4 before the last periods date we are not planning other child yet and use pull out method and didn't face any mistake so worried about the delay what to do yes I used 5 to 6 entamezole and gravinate tablets this is the only change happened what can be the possible reason please help last period date was 1 July expected was 28 July according to the cycle