What Causes Delay In Periods?

Hello sir..Last month on 18 i and my girlfrnd had foreplay..We dint have intercourse but yaa i rubbed my penis there..Her periods was scheduled to start from first week of dis month..And still she hasnt got..She says it pains her in her lower abdomen like it use to happen in her periods..But there is actualy no flow..She says when she goes to washroom then some drops can b seen..Is there anything to worry??