My theory is that you are lactose intolerant, and this protein is causing you GI issues. Many times a bad smell from the mouth can be an indicator of GI issues, stomach problems, and tooth health. Therefore, if your teeth are alright, then most probably it is GI due to lactose.
What you want to do, is get a protein product that is made with whey isolate, not concentrate. The concentrate has lots of lactose in it, so proteins made with this sort of whey will cause stomach issues. On the other hand, the Isolate is the purest form of whey that has no lactose in it, so it does not cause these problems. The best one I know is the Isolation by N2Bm - it is made with 100% whey Isolate, has no additives, or sweeteners. Besides, it tastes really great and has a good price, so it is well worth considering for use.