What Causes Missed Periods, Nausea, Headache And Vomiting Sensation?

I am baffled. My symptoms are missed period, nausea when I eat, starving at times then I eat and get full quickly, headache, quessy, mid left side back pain, heartburn the last week along with indigestion. I took a pregnancy test which was negative. I do not feel I am pregnant and I have read about phantom pregnancy. For some background: In the last year, I dropped an excessive amount of weight in 2 weeks and lost my period for 9 months, since it has been back it has not been regular in length, flow, pain, etc, but never has it been longer then 27 days...going in 35 days. I also had a litany of tests that my endocrinologist conducted...all normal as well as my OBGYN did ultrasounds and a cervical biospy...all negative and inconclusive. Any thoughts on the current symptoms and what is could be?