What can I do ? I missed menstruation, I have stomach pain , nausea , flushing, and pain in the lower back since I woke up this morning.

Without much more information is difficult to offer advice here. A missed period could be due to a number of conditions including pregnancy , ovarian cyst , excessive exercise, acute stress , illness, obesity, thyroid disease , pituitary problems , and certain medications (eg Hormones or antidepressants) . If this had sex without using protection last month , the first thing you should do is take a pregnancy test . If is positive , see your gynecologist to discuss your options and / or to start your prenatal care. If the test is negative (and has not started her period since I asked this question ) , see your doctor to be able to make a medical evaluation. I hope that helps . Good Luck!