What Causes Delayed Periods With Negative Pregnancy Test?

hi i had sex on 1st week of march after tat i had my periods... i forgot whether i had 2 regular periods r 1 .... now june 20 my prriod should come but not yet started... i m having lower abdomen pain .. i took 1 perg home test it was neg on 14 days of missed period .... m i preg