Re: Low blood oxygen reading...
Was the blood drawn from your wrist? The test you had drawn was probably an ABG (Arterial Blood Gas). It tells your physican your blood PH, your CO2 (a gas exchanged in the blood through respiration) your PO2 (the oxygen content of your blood) and your HCO3 (Bicarb).
If your PO2 is low, it depends on lots of things. One is your age, any health conditons ( if you have been dx with COPD, CHF..ect.) in which you might have, and sometimes just plain ol error. Your PH is usually an indicator of how acute or chronic the condition may be. If it's really low and you didn't "fit" the way a typical person looks with this X blood gas, chances are they could have gotten a venous stick. You have two types of blood, BTW.
Also, that "thingy" they put on your finger is pulse oximeter and it's basically a non-invasive tool that is used initally to determine the oxygen content of your blood. It isn't a perfect tool and will sometimes produce an incorrect measurement. There are times when it shouldn't be used at all, but that's a different subject.
Hope that helps.