Several of your relatives have “already” guessed or suspect that you’re pregnant. Most likely they will congratulate you/wish you well and either say that they suspected you you were pregnant…OR…they will feign surprise. It won’t make a difference to them who comes out with the news first.
This is your mom’s first grandchild, she’s excited. When you know of exciting news, you naturally want to share it with your family/loved ones. You’re bubbling up with excitement to share the good news. She probably wishes to talk about her various plans for the baby (clothes, etc) with the other aunties. It’s okay, Nadz. You have lots to be grateful for. The scan turned out okay. The baby is doing fine. You’re doing fine. You have a husband who loves you. Your baby will be welcomed into this world by a loving family/grandparents/an excited grandmother. You and your husband have the means to support the baby. You have a roof above your head, food on the table, and your living in dignity. Think about those who have far less (child out of wedlock, no means to support it, non-supportive parents, living in poverty, severe health problems, etc). In a way your mom is right. Some relatives will complain that you didn’t tell them about the pregnancy.
A first child tends to get everyone excited. With the subsequent children, people mellow down 
Calm down, focus on the positive, count your blessings. You’re pregnant, it’s not good for you to be so (unnecessarily) stressed) in this condition.