Fullness of abdomen in medical science is typically seen in Fluid, Flatus,Fetus,Fat and feces. I hope you take it in a fun way. In your situation since you had copper T inserted the chances of Ectopic pregnancy(tubal pregnancy) are more, which dont have the characteristics of normal pregnancy and is even dangerious. Since you are in your fertile age I would suggest you to have an Ultra sound
of your abdomen done. It will rule out any pregnancy related changes and if any other reasons about the pain.Would suggest to get checked by a Surgeon if all this comes to be normal. Hope my answer help. Please ask further if you have doubts.