Ultrasound..what does it show

My GP ordered an ultrasound for the area around my ribs and abdomen that has been hurting so bad. He was concerned about it being my gall bladder. Well, it came back clean. What exactly does ultrasound show? Would it pick up an ulcer and can it show Cancer. My Oncologist hasn't been concerned because my CT and PET were clean. But my side has been hurting for awhile. Has anyone else had this pain around that area, I'm beginning to think its the Herceptin. HE put me on Nexium and said if it it kept hurting we'd look further. I'm beginning to think I might have an ulcer or something of the sort..sherryg683