Mum has chronic kidney disease and personality has changed

I care for my Mum who has chronic kidney disease. Not yet on dialysis but she is feeling very unwell every day. She is usually upbeat but she has recently changed and is irritable and very snappy with me especially if I show concern for her well-being or try to help her in any way. She will not talk to me about how she is feeling but she will talk to my sister on the phone. It is like she is shutting me out and treating me like an enemy. It is very upsetting for me. She is pushing me away and it feels like she is turning against me when all I ever do is try to help her. We used to be close but I feel a distance now. She doesn't want me asking how she is feeling. When I ask her she snaps at me and tells me not to take any notice of her and not to ask. I explained to her that if I don't ask then it will seem like I don't care about her. She says she is sick of herself and that I must be too. I tell her that is not true and I am not sick of her. Has anyone else gone through this kind of thing as a carer? It has happened so suddenly and I don't understand why.