How did my mom suddenly become so selfish?

Hi everyone, I'm struggling with my aging mom and was really hoping to get your thoughts, opinions, advice, and experiences.

I am the youngest of three children. My siblings are both married with families of their own. I, however, am single and recently moved back home with my aging parents. Sadly, my dad unexpectedly passed away this year. It was extremely difficult for all of us—especially my mom—but we all leaned in on each other to help get through it.

As children, my parents ensured our family was filled with love and compassion. My parents have always kind and considerate people; they tought us how to be good people who always think of others.

Recently, however, my mom's personality seems to be shifting. To be blunt, she seems to have become quite selfish. When invited to spend a few days with my siblings and their families, she makes excuses to explain why she can't be away from home for too long. Instead of spending time with us, she would rather spend hours on the phone gossiping with her own siblings and friends.

When we brought it up to her, she became angry and defensive. Instead of talking to us about it, she shut down and said some rather hurtful things like, "At this point, I don't care what people say or think."

We love her and just want to spend time with her. Everything we do for her is out of genuine love... not out of some sense of "obligation" as she seems to believe. She mischaracterized everything we said. She claims that we're just going to get "fed up" with having to care for her, but nothing could be farther from the truth. We made this clear to her, but it's as though she's not even listening.

What is going on with my mom?? Is this some form of grieving my father?? I'm so concerned and don't want to lose my mom. I appreciate any and all feedback.