What has happened to my sweet little Issie?

I can not figure out how this happened. It seems like overnight my sweet little content and quiet baby has turned into a schreeching, yelling, throwing herself on the floor in frustration little crazy girl. At first I thought she was just exploring her vocal range, but now it seems like almost a constant scream and squeal and screech that is seriously like nails on the chalkboard to me. She started hitting her sister and throwing toys when they don't do what she wants them to do. I thought this happened closer to 18 months or 2 but not at 14 months! I have been trying the sign language as I think the frustration is partly lack of communication skills but she refuses to do any signs but the sign for milk. It has been this way for a few days now and it is not good. Any suggestions to save my sanity?