Alien Living In My Tummy?

Please help me! I have a weird lump appear and disappear in my stomach a couple inches above my belly button. You can actually see and feel it going up and down. It reminds me of when I was pregnant with my son, he would press his foot or fist up against my stomach and we could see it protruding from my belly. I am 5’5” and 115 lbs. I thought maybe it was because I am so thin, that maybe I was watching my intestines contract. Then I thought “oh no, do I have a worm growing inside my stomach?” I had my stool sampled, but nothing was found. I have questioned my doctors about it, but they have no idea what I am trying to describe. Please help!