FEELING UTURUS? PLEASE HELP

When you feel you uterus does it float around in your stomach... Like does it sometimes go right on the other side of your stomach in line with your hip bone and then go back into the middle. I dunno if I'm pregnant or not. If I am if be 20 weeks. I still have periods and negative tests. But when I feel around my stomach I can feel like a movable bumb... 
I'm so worried I'm pregnant but I have had negative hpts every week for 17 weeks now using morning and afternoon urine. 
I don't know if I can feel movements as I'm always quite gassy in the stomach. Please help??