Please help , My grandmother is ashamed of me and pretends that I'm her daughter and my mom let's her ?

My mom had me when she was 13 and I guess my grandmother doesn't want people to know that because she's ashamed so she says that im her daughter. But it really hurts that she's ashamed of me and my mom. But my mom doesn't seem to mind either which makes it worse. We could be out having a meal and then my grandmother would say these are my two daughters. All her country club friends have no idea that I'm her granddaughter. Its just all messed up. What's your opinion on the situation and what do you think I should do about it.