If you had abdominal scans and the lump is on your back, it may not have shown up or been seen during the exam.
"I am a plus size and was wondering if it could be seen in a ct scan of the abdomen or if I need a scan of my back"
Hi, an ultrasound depends on seeing the difference between the lump and the surrounding tissue. If a lipoma is in a layer of subcutaneous fat, then it's harder to see - especially as incidental. A CT might very well be similar, since a CT sees density and the density will be similar.
Also, as backhurtz mentions, a CT isn't full body so it depends if the CT slices were at the same spot as the lump.