My 6 month old has a rash everywhere but the diaper area. It's not the detergent. What else could it be? Is it possible her new clothes caused it?

Many possibilities. Think about whatever touches babies skin in those areas that does not touch the diaper zone. The sizing (color stabilizers) in newer clothing, the fabric softener or dryer sheet, lotions or ointments you might apply, etc. You often figure this out when you exclude allot her possibilities.It is unlikely to be a food issue as that can effect every skin surface.