There are a few possible angles to this.
It's very possible you ARE being overly sensitive, as you're in a big state of flux emotionally and physically. You may have lost one of your coping mechanisms with food no longer being as available;
Your SO has probably seen you fail time and again on diets and they may be afraid you are going down that path again. We didn't generally start eating off plan on previous diets all at once, it usually starts with a bite here or there first, then get's out of control;
Doing this in front of your nephews probably added a level of embarrassment that wasn't necessary, making you both a bit more prickly for each being corrected in such a public way.
Your SO and/or you may also have some fears about how you losing weight may change your relationship.
WLS tends to highlight both the strengths and weaknesses in relationships. You are changing the status quo.
In the comfort and privacy of your home have a calm discussion about everything. About how your SO has been supportive and how you appreciate the support. About your plan and what the details, depending on how involved you want your SO to be. About the level of policing you would like or not like, even how policing (if appropriate) should be done, specifically privately and any other features. Talk about fears and make reassurances as you are both on the same team. Reinforce your positive feelings about each other.
This discussion may happen more than once, or in different ways. It's OK.
Good luck,
Tek