Thanks for the query.
There is possibility of some pelvic infection
Sometimes early infections may not show any changes in ultrasound
So you better take one course of antibiotics to treat the infection, which can possibly decrease the pain.
And also you have to rule out other possibilities of pain abdomen like:
For more details you can ask me through: http://www.healthcaremagic.com/doctors/dr-sree-gouri-sr/63429