I don't know what other tests they can do. Have you had a HIDA scan to check your gall bladder? Those can swell up and hurt, but if they don't have stones, they won't look abnormal on an ultrasound (even if they are).
Does anyone think you might have a twisted, looped, strangled small colon, or that it's herniated? That's not terribly uncommon and is bad to cause constipation. Surgery can get it straightened back out or fix it if it's herniated.