Indeed, if it is as said, you really do need to see a doctor as soon as possible. I experienced an inordinate weight gain (I'm not sure if it took a month or how long it was - I don't weigh myself that regularly), but it was on the order of 20 pounds and was the result of a pretty serious problem.
[edit - naturally "you" isn't you, Ben.]
[edit - it did put me into the hospital for about ten days.]