Hmmm, it's too bad that you have to deal with these comments - I agree, it's pretty annoying. But I feel that one of the reasons that your friends are claiming you have a "bad attitude now" is maybe because your response to them can come off as pretty prickly. As irritating as these comments might be, if you want to keep these friends (which, of course, you may not! and that's totally understandable), you'd have to entertain their stupid questions and be a bit more patient. For example, this is how I'd respond to these annoying statements:
Annoying friend: "You've lost your shape!"
You: "One of the reasons I had weight loss surgery is because I didn't like my old shape...my new shape fits into an awful lot more cute clothes! Haha! *change of topic*"
Annoying friend: "You don't need to get any smaller!"
You: "This isn't so much about size for me, but more about health. I love how much lighter and more energetic I feel now that I weigh less, and it's so much easier to walk and run without all that extra weight! Did you wanna join me for a 5-hour hike in the mountains this weekend, by the way? *friend will change topic herself to get out of this offer*"
Annoying friend: "You do not need to go to the gym that's why you had surgery!"
You: "Exercise is very important for health and well-being. In fact, it has been shown via research that overweight people who are active tend to be healthier than slim people who are sedentary. Not only is it great for your cardiovascular system, but also your mental function and your moods. Would you like to go for an hour-long run with me this evening?"
Just some ideas. If you respond in such a manner that doesn't antagonize them further and is entirely logical (why wouldn't you exercise when it's so good for you!?), they'll just stop making those comments. And if they don't, then maybe it's time to find some new friends.