What Causes A Delayed Period With Negative Pregnancy Test?

i am 22 years old,i got my menstrual periods on 8th of april.my husband came from abroad on 9th.my periods got over on 15th and after that we had intercourse everyday.now its may 19th and still my UPT shows negative.what is happening doctor?we both are really in tension,why its happening at this early age?