What Causes Missing Periods And Negative Pregnancy Tests?

Hi,
I haven't had my period since the end of February, I am not pregnant. Also I have been trying to loose weight and its usually not that hard for me but lately I can't seem to loose. I feel tired all the time, I feel sad and angry for no reasons, I have been feeling like I dont know who I am, I think im someone, but then I want to be someone else. I want to feel happy with myself but i can't, I feel moody most of the time.