Why did my kitten die?

I came home last night to find my kitten laying dead in her kennel. We rescued her and another cat 3months ago from the shelter and hadn't yet been able to get her shots, although the other one already had his before we got him. She seemed perfectly healthy, very playful but was pretty skinny for her age and still as tiny as when we brought her home. She did have worms when we first brought her home but we dewormed her as soon as we found out. Ever since then, we haven't had a problem with her except for her being so small still. My husband buried her last night and we ended up giving the other cat, who was still alive away. When my husband buried her, he said her body felt really stiff but her neck felt broken. The only thing we can figure is that the other cat accidentally broke her neck cause they were sharing a kennel and there were numerous times we had to yell at the older cat because he kept jumping on her and trying to lay on her, not realizing she was so much smaller than him. He wasn't ever vicious so I doubt he tried to kill her. Could you help me understand if this sounds plausible that he could have killed her or whether her non growth pointed to a more serious problem. We don't have any money to take her body in and find out what killed her and I'd just like to get a professional opinion. Thanks. I would greatly appreciate it.