Help? Knee Injury? But no Limping??

Note: tomorrow morning I am calling the vet and taking her in! But what I am going to describe happened just recently and no googling I am doing is giving me a good answer. So I just want to hear anyone's ideas or experiences.

Holiday, my 2.5 year old, slender, muscled up, healthy Golden Retriever female is experiencing what I am certain is knee/stifle joint pain.

Here's what's up:

1) She has a very straight rear (and front) which does tend to lend itself in my personal experience to ACL tears and injuries. She runs and plays hard and turns and twists in awful looking ways while rough playing.

2) She is NOT lame. She holds the leg normally, walks on it normally, and her gait is not off at all.

3) She has slowed down the past few days. She walks slowly or lags on the leash, and runs less even though she visably wants to play. I have been suspecting she could have mild hip dysplasia, but was going to give it a week or so in case she simply pulled a muscle or something, but then...

4) Tonight I saw her chewing her stifle joint. There is a raw spot there, which I have treated. She has NO skin issues, I never see her scratch or chew herself ever. She has no allergies and she has no parasites. I am 100% POSITIVE that joint hurts her and that's why she was licking/chewing at it.

I have her a rimadyl for tonight and she's asleep beside me. She hasn't got any issues jumping on the bed or anything.

What the heck could it be? Can a dog injure an ACL without it being serious enough to cause lameness? Could she have bad patellas without lameness or the "popping" that dogs I have known with PL have shown? Ideas, thoughts? Thanks!

Here's a picture that shows her admittedly poor structure pretty well.