Hello, I Noticed That My Stomach Looks Like Its Beating..

Hello, i noticed that my stomach looks like its beating.. I am a very thin female, when i push my stomach out it gets very big, almost like i just finished a holiday meal... When i stand up the lower half of my belly gets bigger amd pops a bit.. Ive taken test and theyve said negative and i have an iud.. It feels like its bubbly sometimes, and it kinda moves around sometimes its solid in one spot other times its not. Could i just be over paranoid and watching my food go through my organs? Could something be wrong or is there a chance that im geyting false tests?