What Causes Nausea, Sickness And Lower Back/abdominal Pain While UPT Is Negative?

hi um ive been sick on and off since may. and it first started off with me having neasua and sick to my stomach feeling, but then it turned into me throwing up constinely when i ate or not. i even took about 5 home pregnancy test n had a doctor do blood work and everything came back negative. but now in the past month i have been neasua again, feeling really tired even when i dont do much, also have this really bad pain in my lower left abdomin area, and my headaches are back along with lower back pain and ive had rapid wieght gain in the past few months that has made it hard to fit into my clothes.