I agree with djzen, she may be going out with her friends more and more because maybe she feels like doing something different? This could be a reason why though:
"we spent most of those 9 months basically joined at the hip" - as you said..
Your relationship is only 9 months old and you've been joined at the hip the entire time? With a relationship like this you should be going out, doing things that boyfriends and girlfriends do.
She may well be going out with her friends because shes felt like shes lost the feeling/ that rush you get in a new relationship, when everything is exciting.
My advice to you is, take her out places, for a meal, or out on the town; something you two can do together.
Surprise her aswell, If your going to take her somewhere, dont tell her where, or buy her flowers / chocolates.
If you two are old enough, then go down the pub, have a laugh; and what you'll find soon enough is that shell go out less with her friends, or may even invite you along.
Keep her keen dude
