What Causes Movement Feeling On Belly Button On Right Side?

i have been feeling movement above my belling button on the right side. I have no pain but i have been feeling like i am going to start my period. I took a pregnancy test and it was neg. I am on birth control..implannon. i don't know what is going on. i feel it all the time and it is not going away. it feels like something is really moving in there.