I feel miserable . I dont know whats going on. HELP!

I been late on my period for about 2 months now, and i have taken 2 pregnancy test and they all come back negative. I have been extremely tired and have not been feeling well at all. I tend to get pain in my back and lower abdomin every once in a while, kind of like mentrual cramps. I havent had any bleeding at all so could i still be pregnant even tho i keep getting negative results? i had regular periods before that.I feel miserable and often nauseas. I dont know whats going on. HELP!