Whatsapp Hacked

On whatsapp ,in my group my friend send message to the group and her message appearing to me by
Strange number but other friends appeared to them by her name and I checked participants and I found all my friend and this strange number but when I checked participant on my phone friend there is only our friend and this strange number is not found and after few days the strange number left group and also appearing to me only he is left group
So I think my whatsapp hacked
My question
1- is that mean my whatsapp hacked?
2- How can I know him?
3- How can I prevent him from hack my whatsapp?
4- And any benefit information in this subject tells me please.

Thank you very much and I am sorry about my language because E is not my mother language
and I still learning E.

Thank you!!!