I've been diagnosed with secondary amenorrhea, had a pelvic utrasound-normal. haven't had a period in six months! what can be the problem?

Amenorrhea: Secondary amenorrhea can be due to many things: pregancy, chronic stress, long-distance running, premature ovarian failure, an abnormality of the pituitary (eg high prolactin levels), endometriosis, scarring from pelvic infections (asherman's syndrome), autoimmune disorders, etc. Next step would be to check your hormone levels to see if they are cycling normally.