There was a book several years ago called "Fat is a feminist issue" that proposed that being overweight made women more "substantial", literally taking up more space. This could lead to feeling more powerful. Also, being overweight could act as a protection against unwanted male attention; allowed one to hide so to speak. I have found being middle-aged has the same effect--young people, especially men, look right through you!
I noticed when I was in a more traditional office setting, in a managerial position, being overweight and dressing less feminine did make it easier to deal with other male managers. When I lost weight (unfortuately its back), I got the "wrong" kind of attention from my male colleagues.
So...perhaps now that you are thinner, people are noticing you more, and you are feeling the difference in how you are treated, resulting in unease. Once you get more used to your new body, you will get your confidence back.
Just a theory....