13 pounds in a week? Is it on the same scale?
I don't think that's even possible! Have you tried a different scale? Sounds to me like that one's broken

That can't be right-a weight gain or loss of that much would have been noticable to you OFF the scale, as in you wouldn't have been able to fit into your jeans last week.
Seriously though, weighing myself at the doctor can fluctuate for me as much as 5 or 6 pounds from my scale at home, because at home I only weigh in the morning with no clothes. Depending on clothes, shoes, what you've already had to eat and drink that day, scale calibration, etc. fluctuations can get kinda wild. You can't let it get to you.....if the number on the scale is important to you (it is to me, so believe me I'm not judging

), get a scale for your bathroom and use it at the same time of day each time you weigh, either no clothes or the same kind. If you still have such a high fluctuation, I would DEFINITELY be concerned about what's going on and ask your doctor about it.