If shingles was the whole time localized to left flank and left back, but then on day five it spread to front abdomen, is this a bad sign?

I don't think so if the new eruption was only on a line wrapping around the trunk& didn't spread past the midline of the abdomen. This means the viral eruption involved only one nerve root that leaves the spinal area and wraps around the back,flank and only one side of the abdominal wall. I'd call your case one of classic shingles which can be treated but is very uncomfortable. I hope this helps