"hate being a mom"

Well my mom has always had a ton of issues but wouldn't admit to them. She came in the house today crying and said "sometimes I hate being a mom. You want me to be a piece of SHIT fine I'll be a piece of shit." Apparently she's mad at my younger brother for whatever reason. 
Am I wrong for just not saying anything and walking away? I still live with her and have a one year old. 
Also, should I take what she said to heart? She's said more things worse before. And wasn't a very nice mom while I was growing up. 
If I say anything to her right now I know she'll go off on a shpeel about random stuff and drama I don't really feel like dealing with. What should I do? 

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