What could this dream mean?

Could mean anything but perhaps she is missing taking care of you and her dream was about her revising a time when you were dependent on her.

That’s really interesting. I’ve had a hard time adjusting, but I think it’s even worse for her. She even cried for a while at the beginning realizing her whole life had been dedicated to raising me and my brother. So I can definitely see that now.

That was revisiting a time.

Going back to her childhood place may have to do with making her feel safe in a familiar place.
You in her arms means she was holding you safely.
Peeing in her arms mean she may feels she is holding you a little too securely and she needs to let you learn to walk on your own and is having a hard time recognizing that your not a little baby needing to be craddled.

Anxiety.

“To dream that someone is urinating on you means that you are feeling the emotional burden of this person. They are dumping all their feelings on you. Alternatively, the dream symbolizes your lacking sense of self-worth.”

Dream Dictionary

The dream could express your Mom’s internal guilt, for not taking AS GOOD care as you as she THINKS she might have. Perhaps in some way she believes she has done harm to you and she resents herself.

I’m not good with dreams but maybe she’s feeling you hurt her a little by going away? Her baby grew up, peed on her and left?

“piss on you” is an expression to mean that the person ( your mother) may feel that you could care less about her missing you?
Your mother may have had a symbolic dream involving that expression?