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The causes for absent periods can be many. These include pituitary hormone derangements, thyroid problems, anorexia, severe illness, high androgen levels, PCOD etc. Look out for other associated symptoms like change in breast size and breast discharge, headache, weight changes, increase in acne and hair growth (male pattern), dryness in vagina and voice changes etc.
If pregnancy is suspected (as suggested by the persistence of symptoms), then it should be confirmed by getting examination and blood HCG level tests done by your gynecologist. Urine pregnancy tests can show false negative results at times so they cannot be relied upon for accuracy.
Do not delay your visit to your gynecologist for further evaluation and investigations (hormonal levels, CT scan or MRI of head, endometrial biopsy, ultrasound scan of pelvis etc) to confirm the cause and start with appropriate treatment based on investigation results.
Visit these links for further details:
http://www.nlm.nih.gov/medlineplus/ency/ar
ticle/001219.htm
http://www.estronaut.com/a/secondary_ameno
rrhea.htm
Take care.