Suggest Remedy For Delay In Periods

Hi, I am usually pretty regular with my period, every 28 to 30 days. I am 12 days late right now. I have taken 3 pregnancy test and they are all negative. This past week I was feeling a bit nauseous and with loss of appetite, but now I am eating normally but have had some diarrhea for the past 3 days. I have been told I have been kind of moody lately. Could I still be pregnant, even after the 3 negative pregnancy tests?