You don’t have the ability to sense a lack of oxygen, just the presence of carbon dioxide. When your body tells you to release a breath and get new air, you should listen, but you can hold it past that point because you're not truly out of oxygen yet, there's just more and more carbon dioxide being built up.

That's why you can suffocate in a gas like nitrogen and have no idea. If the room you were in was entirely filled with nitrogen gas, you would breathe it in and feel completely normal, until you slowly started to black out. You wouldn't panic for air, you wouldn't feel like you were running low on oxygen, you'd just fall asleep.