I don't understand. My girlfriend used to be so active and fun. We're together now for 9 months and it looks like he doesn't want to do anything fun. She just wants to be with me, sitting on a couch. I like that too, but sometimes I want to do silly stuff with her, like playing pranks on our friends, or going out but somehow she always "it's something we shouldn't be doing for our age" or "you're so childish." What happened? I remember she used to be different, her friends keep telling me she changed as well, in a very negative way.

Your girlfriend could not have changed if she is not being influenced by someone else, or something. Sometimes, new people or new experiments in our life touches us in a way that impacts our own behavior. Maybe if you share your own concerns with her, she will be more aware of her own changing habit.
And remember to stay understanding to her new view point, she could actually be troubled and need someone to hear her out. I think that you should speak your mind, and if she is willing to talk, than listen.

If she doesn't want to prank your friends perhaps she is maturing at a faster rate than you are maturing.

The only way to know what's bothering her is to ask her - but never ask a question if you aren't prepared for the answer.

Play time is over, and its time to talk and be willing to listen. 9 months is still a learning time for couples.

You want total strangers to tell you if she has changed? You have just told us that she has changed, as have your friends.

Then you ask - what happened? How would we know? The whole nature of the question is immature. As you grow you are supposed to be able to talk to each other.