As CatherineCherub's link says, if the sample is disturbed or mis-handled in any way it can result in an over high reading. My husband had a sudden high reading, which the doctor took one look at and said it was almost certainly wrong and to ignore it. A fresh sample was taken and this came out as normal. I think this is all too common, so don't worry, until there is something to worry about - which, on this occasion, there probably isn't.
Sally