What would cause a major rash from sholders to feet. If u have not changed laundry or bath soap.?

Any other exposures? If your rash is truely localized you want to consider things you got into. Detergents, fabric softeners, dryer sheets, etc. Any body wash you limited to those areas or lotions, etc. If the skin of your neck/face has any rash at all, not necessarily identical to other sites, an ingested product-food/additive/meds could all be an issue. Occasionally a viral process will produce such a rash .