I would say that it is jealousy. I had a friend that lost 10 stone, and I was a little jealous, but interested to find out what she had done and how I could do it too. But one of our mutual friends was almost mean. She never congratulated her, made snide comments and acted as if she was still hugely overweight. I think it was because she didn't like the fact that our friend was now the slim one, and slimmer than her.
Relationships can be difficult, and often you take the place of the "fat" friend, and your friends see you a certain way. When you begin to move away from this idea they hold of you, it can be difficult for people to adjust and it can cause some resentment, i.e. your moving on and leaving them behind, kind of thing.
I suspect that they will come around when they see how well you are doing, and how much the weightloss is helping you etc.