The human body can only detect a buildup of carbon dioxide in the lungs, not a lack of oxygen. This is why it’s uncomfortable to hold your breath for a long time. If you inhale pure nitrogen while being able to exhale, there is no build up of CO2 and therefore little to no discomfort.
Wikipedia cites a USAF text, saying: “Some individuals experience headache, dizziness, fatigue, nausea and euphoria, and some become unconscious without warning.”