Sounds like some form of allergy to me.......if not the shampoo,maybe food??
You say that she gets very dirty.....from what???? Does she spend a lot of time outside??? Perhaps this is something outside???
If she is not rolling in something nasty outside or digging a lot and getting dirty in that respect , I would be rather worried
Also, does the smell eminate from her without an external source??
Please write back with more info, I am very curious