My friend has missed 3 months and she has taking pregnant test 2s and says she not pregnant ? why is this??

Many causes: Woman can miss periods for many reasons. Some women have irregular periods. Other woman may have regular periods most of the time and will miss having a period for 2-3 months for no apparent reason. There are many things that can cause a woman to miss a period, including excessive physical exercise, being too thin, being obese, thyroid disease, anemia. Perhaps your friend should be evaluated.