This girl I really like will seem like she is obbsessed with me one day and the next day just completely ignore me does anyone know what this mean?

I think it probably means that she likes you, you should ask her out. If she sais no, don't worry about it. But it seems to me like she likes you.

And speaking as a girl, she might feel a little nervous at times if she's having 'mood swings' around you.So don't worry because I think you'll do just fine :)