What Causes Flutters In Abdomen With Negative Pregnancy Test?

I am 32 years old and a mother of 5. The last time I had sex was on July 15th. I had a normal period August 12-15th. That was the last time Ive had my period. My whole life I have had a perfect 28 day cycle. Ive taken many pregnancy tests, all negative. And now I am feeling lots of "phantom kicks" and rolling in my belly. It doesnt feel like normal gas bubbles, as you can feel it from outside. I am also tired 24/7. Im 99.9% sure I am not pregnant, as Ive been pregnant 5x and have VERY obvious early symptoms. What else can it be??