Could I need a knee immobilizer?

I am a 15 year old girl who plays varsity basketball. I hurt my knee yesterday and now it is a little swollen, it pops when i bend it about 90 degrees. It hurts really bad. The swelling is around the bottom and to the left of my right knee. I went to my school trainer and she just said it could be an ACL sprain which I just don't believe because it has been like this for about 6 months.