For people to change?

Hello, I am writing this because i am quite interested what everyone has to say about this :)

Right i have 2 friends, who are/were my best friends; Friend A and B. Both these people were very close to me and helped me in my life a lot. Friend A is a boy and friend B is a girl. I have known Friend A for 4 years now and he has been the nicest person i have ever met in my whole life. Friend B i have known for just under 2 years now.

Friend A and B were also best friends, or probably a little more about a year back, but a few months later they had a really big argument and since then they have both stopped talking to each other. I met friend B just after Friend A met her and me and Friend B became great friends, and very fast. Friend B got so close to me that i couldn't not be with her all day. She helped me when i was down and even stayed up late nights up just to talk. After Friend A and B stopped knowing each other, friend B was still fine with me. She always talked to me and always wanted to know how i was doing. However, We all broke for holiday for 6 weeks and then on the day we all saw each other again She changed with me completely! I had not seen her for 6 weeks and i said 'Hey!' Several times and she just ignored me. I just took the impression that she did not hear me but then for the whole of that day she didnt say a word to me. A week later i noticed she stopped texting me and saw that she obviously didn't seem to want to talk to me. I was really upset and i couldn't stop thinking if i had done something wrong to make this happen, when we would actually used to talk about how amazing our friendship was. I always see her talking to other boys but just ignoring me, she doesn't bother coming upto me or even saying Hello anymore. I became really upset and even angry at her sometimes knowing the fact that she can actually see the pain she is causing me but doesn't do anything about it.

Is it normal for people to do this? Or am i just a very delicate person? :/