I think your weight gain sounds like a hormonal imbalance
issue. You must either be on steroids or your endocrine
system is wrecking havoc and throwing a spanner in the works and
causing your weight gain.
That is, unless you've been gorging yourself silly, in
which case, the solution is more obvious. 
I think this paragraph just added another layer of chin to my
chin!!!!
I hope its you are just saying that its water
retention......