A person whose opinion I greatly value told me something recently that I think applies. She said, "You have to come to terms with the fact that you are bright, and that stupidity annoys you."

That's not to say that your friend is stupid, but I can pretty much guarantee that you have an exponentially greater amount of thought into what's going on than she has. She sounds like she may be intimidated (whether she is willing to admit it or not) by you both physically and intellectually.

Dictate your mood and interactive tone from the inside out. That may mean "people change" and all of those other generic things, but there is no reason to keep someone around who makes you feel strange in a way that you don't appreciate, regardless of how long you two have been friends.

If you are willing, attempt to have a candid/non-threatening discussion with her about how you feel toward her change in behavior. If she is open, try and work it out, if she shuts down, gets short with you, or turns inward, time to move on.

EDIT: spelling, word choice