Weight gain only in belly region?

About 3 weeks ago, I noticed that my stomach was getting larger than I'd ever seen it. I've always been thin, and I knew I wasn't eating enough to justify that amount of weight gain. The weight was also only located in my stomach area. I have fluctuated between a flat stomach to having a bit of fat around my waist, especially around my belly button. I have taken 3 urine pregnancy tests and had one blood test, but all of them came back negative. I've been on the depo shot for almost 5 months now, and aside from having a brown discharge with occasional spotting of blood, I have had no other symptoms. There is a heaviness to my lower belly, and my stomach sometimes feels like it's stretching. If I go to pinch to see the fat, all I can pinch is skin. Before when I have gained a little weight, I have been able to pinch and see how much fat was there. I've been taking weekly pictures of my stomach's growth, but I don't want to go to a doctor if I can't get any tests to show me as pregnant. Does anyone know what might be going on? If I am pregnant, the conception would have been at the end of August.