I gained 40lbs in 2 months

I haven't had surgery yet.  But speaking from a normal stomach I can eat tons of junk and not gain that much in a month.  I agree with one person on here overeating and eating junk you could probably gain about 5 pounds a month not 20 lbs.
Another poster said get your thyroid checked. and that was actually my first thought too. 

about 2 months ago, out of the blue, I gained some weight and sure enough, my TSH was very high - as soon as I upped my synthroid - I lost it. 

do you take synthroid or any other thyroid medication?  It is very sensitive and needs to be taken atleast 2-4 hours preferably closer to 4 hours for those that are more sensitive from all iron, calcium, multivitamins, tums, pepcid complete, mylanta, maalox - not waiting the 2-4 hours can totally interfere with the absorbtion of your synthroid.

have you taken any steroids, even a steroid injection can cause severe weight gain. 

and like someone suggested - maybe you are pregnant.  many times people that are completely infertile preop, after losing alot of weight and having the RNY, they unexpectingly become pregnant - this is why surgeons tell us that we need to use 2 forms of birth control for ,atleast 18 months post op and then after that, it is up to you on if you want to become pregnant or not. dv

Get a full thyroid panel (TSH, fT4, fT3, Antibodies).  Also, have your adrenals checked. A combination of high cortisol and low thyroid can cause very rapid weight gain -- been there, done that, got t-shirt....