What is going on with this belly?

Shannon • 35 ttc #3. Have a 13 year old and almost 3 year old. Both girls! #blessed
I'm 18+1. Some days I have a big baby bump, others my stomach is totally flat. I'm worried all the time that something is wrong with her... But then she moves and I feel pregnant again. Anyone else have weird baby bump disappear and reappear?