HI, thanks for using healthcare magic There are different possible causes for delayed period with pregnancy being the most common cause but this was ruled out with your testing. Other causes are: (a)emotional stress or illness (b)high levels of prolactin hormone (c)physical illness (d)abnormal levels of thyroid hormone (e)inadequate diet (f)polycystic ovarian syndrome- diagnosed based on symptoms, blood tests for hormone levels and ultrasound to look at the ovaries (g)side effect of medication The medication should take effect after a few days, two to four days usually. If you continue to have irregular periods then your doctor may need to examine you and do bloods to find the cause. I hope this helps