heaviness in my abdomen is there continuously

I have had a "Heavy feeling" in my lower abdomen for the last few weeks. My periods are very irregular and I haven't had a period in months. I feel tired all day and feel like I have never slept long enough (even if I sleep 8-10hrs at night). I feel nauseous alot too and only seem to feel a little better if i eat something light. The heaviness in my abdomen is there continuously and does not disappear at any stage of the day or night. I did a pregnancy test months back which was negative, but that was maybe 4months ago. I havent had sex since the negative test but have recently noticed this heaviness in my abdomen and my belly appears more rounded than normal- i'm not a small person either, (both of which I have noticed rapidly in the last 3 weeks). What could be going on?