J.
Hi N.!
I understand your concern about your cycle. You are young and you obviously have not had trouble having babies or a cycle. However, there are lots of factors that can go into a hormonal imbalance. None that would cause such an immediate change though, so I would continue to follow up on the pregnancy tests.
Another cause could be recent weight loss or gain, diet changes, or anything like that. Our meats, particularly chicken and soy products have ALOT of estrogens in them. Any excess of these things?
Good Luck. Let me know if I can assist you further.
J.