Ask your doctor if his scale has been calibrated recently. Is it one of those "old fashioned" ones where you move the bar up and down til the other bar is balanced? If calibrated correctly, that is your best reading. It could be that your scale isn't calibrated and therefore giving you bad information, but if it's a regular bathroom scale, I don't know how that could be calibrated.
Also take into consideration other stuff when weighing. Are you "empty" or did you just drink 2 cups of coffee or did you just eat something. Are you clothed? All of these things will change a reading.
personally, I only go by my own scale. It is the only one that I have regular "appointments" with. I only go to the doctor once a year, so that is no help. If I had a gym, they might have a scale that I could use, but I prefer my first thing in the morning weigh-in at home.
Here is an excellent thread on the trials and tribulations of weighing in more than once per day:
http://www.3fatchicks.com/forum/showthread.php?t=51927