Dear helpme,
maybe there is other information you could provide such as other symptoms you are experiencing, visual loss, weight gain, weight loss, vaginal dryness etc. Has there been any recent stress in your life or major changes, bereavement, a house move, job loss, relationship problems etc. Things that can cause absent periods are things such as drastic weight reduction or weight gain, extreme obesity, anxiety, stress, thyroid disease, certain drugs and medication, and most obviously pregnancy which doesn't apply to yourself as you said you haven't had sex yet. There could also be many others. Maybe if you could provide other information it would help point out the problem, I am very surprised your doctor hasn't sent you to a gynocologyst as 5 months is a long time to not have a period.
Regards,
.::suzy::.