At a loss

I am completely dumbfounded. It's like a switch has been flipped in my Myliebel. I have NO idea what happened to her. She has been unbelievably mean and tantrum prone. She behaved so badly at her three year old well check today, that not only do I not remember any of her measurements,the nurse offered to watch Bronson while I took her to the potty for a spank. I was humiliated. I had company over and she went ballistic. She screamed like a nut regardless of what I did and only shut up if I sat beside her on the chair. If I even attempted to speak to my friend she would start up again. Again, humiliated. I have no idea what to do with her. The pediatrician suggested time outs.  I mentioned that to my friend during Mylie's fit. They just laughed. Yeah, that'll work!  I've taken things away.Tried ignoring bad behaviors and praising good ones, time outs, standing in the corner, spanking, and so far nothing works. Girls got some major stamina too. She'll repeat momma over and over while I am holding her and staring straight at her face. Unless I say what Mylie she just keeps repeating. SO annoying. I miss my sweet girl. This new attitude does not thrill me.