What Causes Delay In Menstruation Other Than Pregnancy?

My wife is 2 weeks late starting her menstrual cycle, we have taken 4 pregnancy tests, 3 at home, 1 at the local health department. the first one we took was a home test and it came back positive, the last 3 have came back negative, but she shows no signs of starting her period. Could she in fact be pregnant, or is there an underlying health issue i should be concerned about with her period being late.