I'm gonna have a much different answer than the other people who posted.
I just rescued (stole) a guinea piggy from the clutches of an abusive owner, and my piggy is def. depressed. He isn't withdrawn however, he's very active and very vocal. He used to yell for his life when I picked him up, and frantically walk in circles around his small cage. He's very noisy and active, just not in the way a healthy guinea pig is. One knows when an animal is calling out from stress and from happiness. I'm sure yours is fine, and perfectly happy. My piggy isn't too keen on me at the moment but that's because his past owner was abusive, and he thinks I will be too.
EDIT: Not to mention, my scared piggy eats all his food very quickly when I give it to him, his old owner had a habit of giving him food and then holding him still so he couldn't reach it, pulling grass out of his mouth if he got any, so he thinks I'm just teasing him with the food.
Nope, take your time, it's there to stay little guy.