Welcome to the forum!
It's great that you've come to the forum because you want to help your sister! Perhaps have a private talk to your Mum or Dad about your concerns. They may have noticed some recent changes too.
Trying to start a calm conversation is also an option. This site has tips on talking to someone you're worried about: https://www.beyondblue.org.au/get-support/have-the-conversation/talking-to-someone-you-are-worried-about
I hope your sister can get some help and support if these mood changes continue. Talking to your parents about this is a good way to go ahead.
You are being a good big sibling by worrying about your little sister. It is really hard sometimes to work out if behaviour is due to "normal" and healthy growing up processes where adolescents are taking on the tough job of sorting out and managing their internal world themselves, or whether they are actually sinking under the weight of difficult thoughts and emotions like depression, anxiety , trauma, bullying etc and not coping.
My view is that you can't be the one to have the responsibility to make that call. I would alert a responsible and caring adult to help you talk to your sister and try to identify what is going on and make some plans about how to help her.
If you have parents that you can talk to, definitely that would be the first port of call. If not, try a trusted other family member or then someone in her school community. A good teacher or school counsellor.
You've done a great job being the one to see a possible issue , but you can't fix it alone. She will never hate you for caring enough about her to try and help her.
If you get stuck , call eheadspace or Kidshelpline for assistance.