What Causes Delay In Periods After Engaging In Foreplay?

hi i am 22yr old..i always have delayed periods.it was nearly two months i didnt get periods..i have gained about 8 kgs of weight in the span of 7 months..now i am overweight..my did foreplay with my bf..i checked with urinne test it was negative..i am scared..doctor has prescirbed me susten 300..i didnt tell her about the foreplay..i am worried.. please help