What Causes Delay In Periods And Abdominal Pain While Taking Zinnat?

I have missed my periods. my cycle is 26 days and i expected them to come on 3 of April. i am having consistent abdominal pains and my nipples are hard. i had sexual intercourse on the 10th day after my previous periods which came on the 9th of March.i have been on zinnat drugs. what could be the problem?kindly assist.