Kittens behaviour has changed in last few weeks?

My kittens behaviour has changed quite a bit. We got a new kitten a few weeks ago and after 5 days the boys were getting on great and they still do. But Pumpkyn, my 8 month old has started to change a bit. Suddenly he has become a lot more timid and as soon as he hears the door, he runs and hides. He used to love having cuddles with me, but lately he likes this less and less and most times when i sit to pet him and will get up and move, even if its only a couple of inches. He has also started to claw the carpet lots and has started to poo on the floor regularly, even though his box is clean (although the little one has started to share his box). He also seems to not want to play with his toys any longer and will either play with Gizmo or watch Giz play. He just seems quite down and more scared of people, i'm quite worried about him. Can anyone suggest it anything may be wrong???