Why is my guinea pig not himself? Please help!?

My guinea pig used to be upbeat and lively. All of a sudden, he's been less lively and doesn't do all the things he used to do. When I used to give him a carrot, he would chomp on it happily. When I give it to him now, sometimes he rejects it, which he used to never do. Also, he's been breathing kind of weirdly (not gasping for air.) Is he depressed (he lives by himself)? Does he not like his food (Nutraphase)? I just want my old piggy back! By the way, he just had his birthday...so he's about 1 year old now. Please help.