Missed Period For Nearly A Year, Please Suggest

i'm 26 years old and have had mostly regular periods since I was 12 years old. I have been taking birth control pills for several years and have always been fairly consistent. Last year my father passed away and I missed a few periods, which I chalked up to stress. I got one again about 8 months ago, but then they stopped altogether. I get all the normal PMS symptoms and have taken several pregnancy tests, which of course, have all come back negative. Now I am at a loss as to what might be wrong. What might possibly be going on? I have lost some weight, but nothing significant, my excercise regimen is the same, and there are no other environmental factors I am aware of.