BBT Accuracy

How accurate is bbt actually? I have been taking my temp right after my alarm goes off in the morning for the past 6 days or so and my chart is all over the place. I made sure I got the right thermometer (cost me $15!) and it measures two decimal points. It does take a good minute to give me my temp. Some mornings I am hot just cause we don't have air conditioning (we have a fan going but it doesn't always help) and some mornings I am fine. Could this be affecting my readings? Should I wait till I've mosied around a bit?