What Causes Delayed Menstrual Period With Negative HPTs?

Hi I'm a 23 year old female, and I am concerned with, some symptoms i have been having because i don't know what they are, I have not had my period in a month, Ive taken 4 pregnancy test they all came out negative, i been feeling very tired, and i have been having headaches everyday, I also been having chest pain in my lungs and heart and in my stomach, and ive been gaining weight rapidly.