Welcome to HCM
Your concer is right but pregnancy
is not the only cause you must understand and as you are sexually active it must be ruled out home test is negative but you must visit a Gynecologist and get checke yourself.
of Beta HCG
if negative will rule out pregnancy and then she may need thyroid function and Complete blood count
to see for hemoglobin
and other organic causes and if every other thing is fine a severe form of stress can also be a cause for missed period .
so do not worry please get examined and investigate and I hope nothing serious is there and you will be fine in few days.
Best of Luck