Had a flight today working on my cfi rating. about 30 minutes in during slow flight my instructor asked me "what are you doing?" (i was losing altitude like crazy). I was feeling very lightheaded and my vision was blurred. I mentioned this to him and he about lost it when he saw the carbon monoxide detector was nearly black (supposed to be orange). Luckily there was an airport nearby and were given priority to land. As my instructor was talking to tower I noticed his speech was very slurred, but I didn't mention it until after the flight. i didn't want to make things any worse than they already were. Thankfully we made it on the ground safe, and were picked up by car to return home. Moral of the story, keep an eye on your CO detectors they could save your life.