What Causes A Delayed Menstrual Period With Negative Pregnancy Test?

Hello, I want to ask a query that, last month around on 12-13 june i did intercourse and took 2 unwanted 72 in alternate days and i got my period on 19 june which is 15 days before the usual.after that next month on 10 july i again did intercourse but took nothing as the period should come near to 19 july but today is 26 and still i didn t got my periods. i check thorugh prega news about pregnancy on 20 july it was -ve and again checked on 23 july still it is -ve. Please tell me am i safe ?