What Causes Abdominal Bloating And Irregular Periods Despite Having Negative UPT?

My stomach has grown significantly over the past 2 months slow and steady, but I have not gained any weight. I look like I am pregnant, because only the front of my stomach has grown and it s hard and round, my waist has gradually grown from 28.5 to 34 inches since September while my other measurements are the same as before. I m still getting my periods, but they are not normal. I have also had extreme fatigue, nausea and sore breasts throughout these three months. I had unprotected intercourse on September 3rd and subsequently took three home pregnancy tests but they have all been negative. I have a son who is 11 and when I was pregnant with him I first confirmed my pregnancy with a doctor but had some extra home pregnancy tests at home which I took for fun and it still came up negative at 5 months pregnant. I am not sure if I used the home tests properly then or now. I want to know if I am pregnant or if something more serious could be causing my stomach to grow so much. I ve always had a flat stomach for many years now and this very round belly is worrying to me. I don t use any form of birth control or hormones. I haven t changed my diet or eating habits. I m 31 years old, and I m having regular bowel movements and am not in any pain.