If you have current lab reports indicating your FT3 and FT4, please post them, along with the reference ranges from your lab report, since the ranges vary from lab to lab.
flyingfool is right - you could have become hypo again and need a med adjustment. As to how long it takes for the weight to come off - that depends; for some of us it doesn't.
I believe that both of those medications are pure synthetic T4. So I do not believe that the weight gain is a function of the active ingredient in the medication itself.
Have you had recent blood tests and FT3 & FT4 levels completed. It could be that you have become Hypo and that would explain the weight gain.
The only other possibility that comes to my mind is that you simply had a reaction to the different fillers used in the medication. Some people are sensitive to that.