probably a really dumb question, but

WannaBAmama

ok so I'm not showing yet but when I go to bed at night and I wake up in the middle of the night I have a very defined hard ball in my lower abdomen. When I get up to pee its gone, but every night when I'm laying down for a few hours I feel like I have a bump. This is my question, is that bump I'm feeling my uterus or my bladder? I figured once I go pee it goes away so it just goes back down when my bladder isn't full anymore. I know it's a stupid question but I'm just curious how when I pee my (what I'm thinking to be my uterus) moves back or down  so fast after I go pee at night.