I've had that experience with the synthetic shirts I use for running. That's why I wear merino when hiking instead. I've experimented with wearing the same merino shirt for several hiking days when I have been hot and sweaty. No smell at all. The synthetics seem to magnify bad smells I don't understand why this is. My strategy is take them off and wash them right after running. Can't imagine backpacking with them. I wouldn't be able to sleep in that smell!