Why is my mom so mean?

My step father just brought my mom a million dollar home last year after her constantly crying about how she wanted to move to a particular state. My mom is very nasty to him. She talks to him like a child. He gets on my nerves too but our issues are separate. I’m not a spiteful person that likes to see bad things happen to people even if they wrong me. Lately my mom has been nasty to her son. Whenever you go to her with a simple question she’s always yelling. She acts like a mean old man. My moms mom died 2 years ago and she just lost a friend months ago. I don’t think that it should be a reason to be nasty. It sucks when loved ones lose people because it doesn’t just effect them it effects everyone around them. My mom was very happy last week but this week she’s been very hostel. She’s 50 years old. I actually think that she’s young I just wish that she was nice and happy. Please help?