I'm actually sorry to break this to you, but 'normal' mothers do not exist except in some fantasy television programs.
Every family and relationship has it's own dynamic. Maybe your mom doesn't feel she can call you to ask you about you. Maybe she feels it is easier to call you to ask when she will get to see her grandson (who is your son, so that also means she would be seeing you!).
Do you call her just to 'chat'? And not 'chat' about your son?
I think your brother maybe took it a little wrong when he told you that your mom was angry with you for not having more children. I'll be she would like for you to have more - but only when you are ready. No one wants to inflict children upon anyone when they aren't ready to have them!