What Causes Delay In Menstruation Other Than Pregnancy?

my last mentrual period was on june 17...am i pregnant? but i did home pregnancy test and it is negative result...but i feel dizzy and have runny nose...feel exhausted.....what it is? my ob gyne...prescrbed duphaston 3 x day for 3 days....is it safe...? if possible i am preggy... actually,,i want to be pregnant..as of now..i feel dizzy.....not feeling well....plz. help!