Advice on why she's different to me?

[Vespasunset]

Girl is different towards me than others
I don't understand a girl I know. I have known her longer than others. Yet it seems her behaviour makes me feel bad. She can be very nice kind, friendly and very polite to me often but then other days it's like she less so. For example if we are in a group, and I'm talking she won't initiate nor will she get involved in the conversation, she goes quiet. When I speak spro her she often just giggles, like a polite giggle and that's about it. Occasionally she will initiate something but not carry it on. When I say something funny and others laugh, she won't again get involved and she will just laugh to herself. And if others make fun of me she will laugh also. I have a crush on this girl, so It doesn't make me feel great. Then when I leave the room, she totally changes, she becomes louder, talkative more and laughs at what the other guys are saying. This again changes when I come into the room. When I ask her a question to try and include she just gives me a one word reply but again when I go she becomes more open and loud with the others. It bothers me quite a bit because it feels like I have done something bad to her, and therefore makes me more withdrawn towards her. I smile a lot in general and try and make her smile back which she does quite a bit.
Girls I think her behaviour seems like she doesn't like me much? But I dont know why it changes, should I just let her be?

[Vespasunset]

Anyone?