Pilot DAR, if the symptoms you describe disappeared in half an hour, it might not have been CO poisoning but something else.
This is what I remember from rehearsing this stuff just before my then-girlfriend, wanting to be a nurse, took an exam in it:
The main oxygen transport mechanism in the blood is Haemoglobin (Hb). It has the ability to attract O2 (oxygen) in the lungs, and release it in the organs. This attract/release mechanism is dependent on very subtile variations of the pH of the blood, caused by the absorption of CO2 in the blood (which forms H2CO3, an acid) in the organs, and is released in the lungs.
Hb attracts CO much better than O2, and once the Hb has attracted a CO molecule, this molecule is not released anymore as the acidity of the blood changes. In effect, this Hb molecule has now become worthless to the body, until it is regenerated - I think in the bone marrow. Hb molecules, on average, have a life span of about three months, and that is thus also the time that it takes to get rid of a serious case of CO poisoning.