Your body has no sense of oxygen content of the air. What you sense is CO2 buildup in your blood (because it modifies the pH). In a pure nitrogen environment you can still exhale the CO2, but of course do not replenish any O2. It's not just that you won't notice -- there is nothing to notice. Your body literally lacks the sensory ability to detect it.

To be distinguished from a pure CO2 environment where you cannot expel the CO2 so you will feel the effect of being unable to breath.

After taking one or two breaths of N2, there will be so little O2 in your lungs that breathing will actually run your respiration backwards -- it will pull O2 out of your blood instead of bringing it in.

You'll likely experience a light euphoria, get a headache, maybe feel a bit dizzy. But this will happen so fast it's hard to even be certain -- unconsciousness in under a minute is expected and within just a few breaths is possible.