I had a similar complaint. It turned out to be gas which made me feel really stupid. Now I chew a pleasant-tasting pill when I suffer that discomfort. You may not have the same problem, but trying a chewing a gas pill is worth a try.
Hi and thanks for using the forum.
I am sorry to hear that you have been having this pain for so long. Have you had any other testing besides the ultrasound? If not, you should have a full physical including blood work. Also, you may need an abdominal CT scan. There are many possibilities for the cause of this type of pain including indigestion, gallbladder attack, liver issues, or muscular problems among others.
You should try keeping a diary of the pain and try and see if anything triggers the pain (for example, eating or exercise). This could help narrow down some possibilities.
I hope this information helps. Good luck and I hope you get to the bottom of this soon.
Bridget