I think it's perfectly normal, especially during a back massage, and only in my experience where the client is lying on their front... it's all part of the body cleansing itself. Almost all of my male clients and many of my female clients get a stuffy nose. With my husband, I light an oil burner with Olbas oil in it, to help keep the nasal passages clear. They just need to follow your after care advice, eg.drink lots of water. The stuffiness goes almost immediately after they get up...
If they have no allergies, eg, you don't have a cat, and it doesn't bother them, I shouldn't worry about it... personally.
Hope that helps.