I also wonder if it isn't a little psychological - at the moment, you're probably so mentally focused on feeling full and you've risked so much for this change, it could be your 'lil ole brain wanting to play silly-buggers Try a little distraction if you're up to it and maybe have a chat about it to a close friend or family member who understands. If you like go "go it alone" perhaps you could just journal a little? Start with all of the thoughts you're having, and how it's making you feel. Then consider what is happening to your behaviour.
Once you've done that, go back to the thoughts and ask yourself if they are helping you or hurting you? How can you slightly change those thoughts so that the feelings are less intense (or if you're really good at this, eliminated) and if that is having a change in your behaviour.
An example might be:
Original Thought - If I don't have something to eat I am going to be sick.
Original feelings - worried, hungry, sick, frustrated, annoyed
Behaviour - to have something to eat or drink that perhaps I don't really need
Modified Thought - If I am feeling sick, maybe I need to move around and have a little to drink. If I am still hungry in half an hour or so, I can consider trying something light to eat.
Modified Feelings - still worried, but maybe a little less. Still probably feeling sick, frustrated and annoyed but again I would expect it to be a little less intense a feeling.
Modified behavioural options - I still might have something to eat, but it's something I've considered and given myself time to try something different
Maybe I will have a drink and see how I feel in half an hour
Go for a little walk or phone a friend/ watch TV/ read etc (distraction).
This may or may not work for you but it might just help analyse why this is happening to you if it's not a physiological thing. If you have a psychologist involved they will be able to help you with this (it's a Cognitive Behavioural Therapy approach).
I hope that this helps in some small way!
Good luck and I look forward to hearing about your many successes and new adventures post-surgery!
Cheers, Melissa