My cat eats and drinks but gags?

My cat is eating and drinking, so I don't think there's anything that serious going on. However, he occasionally gags throughout the day. I think it's just a hairball, but he's currently hiding under a corner table. He's been walking slower and is keeping his tail tightly wrapped around him. I do have a new cat, who's a kitten, and he's been adding a lot of stress to the other 2 cats that I have. The other 2 cats are both around 13 years old. Also, when he's sitting down and some one sits next to him, he sometimes randomly lashes out at them. He's still semi active in the sense of that he will move around and jump up, but the entire thing seems kind of forced. All the time, he keeps his tail tightly around him, though. He kinda seems a bit depressed, but I'm not sure if he actually is or if I'm just putting feelings on for him. Any idea what this could be? If this goes on, my family and I are definitely taking him to the vet, but I just wanted to know if it was anything of immediate urgency that anyone knew of? Thanks!!