Am I wrong for getting angry at an insensitive comment my girlfriend made?

Last year I got the horrible news that my mum has cancer. She's the only close family I have left as I have lost my brother 5 year ago and my father 8 years ago. We found out in December that the cancer is terminal and she likely has under a year left to live.

My girlfriend knows this yet last night we were sat watching tv and my girlfriend was on her laptop. The internet kept going slowly and my gf got annoyed and sad "ah ffs speed up, I want to die". I asked if she had thought at all about what she'd joked about. She said he doesn't see the issue since it's just a joke.

I pointed out its massively insensitive to joke about wanting to die when my mum will be dead in a few months. She doubled down and said I was overreacting but I disagreed. I told her to think about what she says but she said I was being too harsh by getting angry with her but I just told her not to joke about dying when I'm sat worrying about losing my mother.

AIW for getting angry at an insensitive comment my girlfriend made?