Queen of tantrums

I am the mother of the queen of tantrums, and she is only 2 years old. My daughter Emma is my whole world. I love her but..... I hate her tantrums. She has gotten the nick name of screaming meme. Emma will throw tantrum anywhere, anytime for any reason. I recently went into a store and my daughter threw the biggest tantrum, I'm talk screaming (not just screaming to hear herself I'm talking blood boiling ear piercing screams) kicking, laying on the floor and hitting. People will look at me as if I don't punish her enough and that I'm a bad mom. I get frustrated that shes frustrated and nothing will calm her down absolutely nothing. Another time she threw a tantrums was at the doctors office it was so bad the doctor asked my sister to step outside the room with her so that we could talk (she cried the whole time when my sister did just that.) I wish I could find the cause for these tantrums and a cure but I'm still going into stores with the fright that I will soon have to leave cause she is so unhappy. This one time at a store it was terrible, one of the workers came over to try and help out by calming her down and of course that didn't work. I have had family members try and step in and that doesn't work. I love my daughter but...... I hate her tantrums.