Dear madam ,I understand your concern.In my opinion there might be urinary tract infection.Urinary infection also presents with frequent urination ,back pain and cramps.As the pregnancy test are negative you may not be pregnant.There might be other causes of delay in period.I suggest you to see a doctor as you might need an ultrasound to rule out any renal stones which might be the cause of urinary infection. If your cycles are regular and this is the first time you have delayed period it might be due to stress the period got delayed. If the same happens next month causes of irregular periods like thyroid abnormalities,polycystic ovarian disease,anaemia can be ruled out by investigations.And once the cause is found with apt treatment cycles will become regular. Dont worry urinary infection subsides with a course of antiboitics.