What Causes A Feel Of Movement In Upper Stomach Under The Ribs?

Hi, I am 20 years old I had a baby 9 months ago and for the past 5 ive been feeling movent in my upper stomach under the ribs, feels just like a baby moving it doesnt hurt but i am worried about what it could be, I had an ultrasound done and they couldnt detect anything I als0 took 3 pregnancy tests and they came out negative, What could this be?