I have experienced something similar and it turned out to be an ovarian cyst. It was nothing serious but they can sometimes be painful and they can be excruciating if it ruptures.
It could be anything - a cyst, an urine infection, ovulation or even appendicitis - sometimes that doesn't always present itself with fevers, loss of appetite, starting from the belly button and then spreading etc.
Its definitely worth getting checked out by the doctor, especially if its that painful its stopping you from sleeping. If its too painful to move, you could always request a home visit from the doctor.