Is this bullying or am I being over sensitive?

On my school bus there is a group of girls always staring at me and they seem to be talking about me a lot...Im really shy and quiet...well anyway, it annoys me that they talk about me but I try to ignore them and today they were saying hi to me but not in a friendly way so I muttered hi to them and then they were like..."geez you are soooooo talkative start a conversation why don you" and they were being sarcastic and then staring at me until I got off the bus. I know that isnt a bug deal but little things like that really gets to me cause I lack confidence around people and I have bad posture (so bad that I cant correct it quickly) what should I do? I also have social anxiety so its kinda hard for me...