HELP PLS! My 3 yo has not stopped misbehaving

Kat ✨

Hey girls! So my daughter has typically been the sweetest little girl ever! She’s smart and kind but I feel like after she found out she’s getting a baby brother she has been acting up! She has NEVER thrown a tantrum and lately that’s all she does. Well today, she threw the biggest tantrum I have ever seen at the store and she would not stop screaming/crying. I was in complete shock and almost lost my mind. What can I do to help her? Might she be feeling left out? I’m so worried this is her new normal. I feel like I’ve failed as a mother. Please no harsh words as I’ve gone through a lot today.