Why does my mom cry after she has been mean to me?

Agreeing with what others have posted both about loss of ability to control their filters and exploring meds to help her mood for her own benefit. My 99-yr old aunt has advanced dementia and says awful things one minute and acts so grateful for my help the next. I grew up with her so I know she was never mean. I just ignore the nasty that comes out of her mouth, literally I don't react to it all, instead I attempt to redirect her thought to something completely different and positive ("Oh, look at that pretty bird on the feeder! Have you ever seen a prettier one?") I find asking her questions rather than telling her what to do helps her feel like she's still in control: "Do you think it's a good idea to remove your glasses so they don't fall off when you put your shirt on?" Etc. She almost always complies more easily, but not 100% of the time.

Teepa Snow has some very good, informative and easy to understand videos on YouTube about how and why dementia changes our LOs and best ways to engage with them to keep them calm and feeling loved and secure. May you receive peace in your heart on this journey with your mom.

Speak to her doctor about her behavior. Maybe there are meds that can help. Is this something new?

Was your Mom "mean" before her Dementia? Dementia has no ryhmn or reason. People suffering from it lose their filters. She may not be able to control what she says. She may still be able to realize that she has hurt you and this causes her to cry because if things were "normal" she would not do it.