M.M.
I am a grandmother of a 3 year old, and I had the same problem with her. Like you said, it pretty much happened overnight, and was very shocking. I did alot of research on the supject, and everything I read said to just walk away, and ignore her. Let her know that you are not going to watch her throw a fit, and definately let her know that she is not going to get what she wants, and don't give in. The next time it happened, just like you said, with the screaming, and kicking, and flayling of the arms, we were in the kitchen, and I told her I was not going to talk to her right now, and I left the room, and went into my living room. Well, get ready, because she started screaming louder, and crying more for awhile. I began talking to her 9 year old sister, and ignoring Summer, and after about 3 or 4 minutes, she moved into the hall way near the living room, and we just kept talking like she wasn't there, and after another couple of minutes, she stopped, and just sat there pouting. I finally walked up to her, and told her that when she behaves like this, grandma is not happy, and she needs to apologize for her bahavior. She did,and I gave her a hug, and a kiss, and she was fine. I had to do this everytime she threw a fit, for about a week, than, Finally, she stopped doing it with me. Believe me, it takes alot of work to keep this up, but it is worth it in the long run. Don't worry, she will still love you, but she will learn that that behavior doesn't work with Mommy. Of course you cannot do this in public. Then I would just leave the store,and go home.