What could be wrong if you missed 2 menstral cycles and home pregnancy test are coming back negative?and she is has sore breast and morning sickness

Missed periods: are very common. There are many things that can cause a woman to miss a period or two. One of the most common causes in teenagers are anovulatory cycles (an egg is released). Other things that can cause a missed period include stress or excessive exercise. A reliable birth control method should be used if a woman is having sexual intercourse and does not want to be pregnant.