Thank you for writing. You are right to be worried. When there is a sudden change in behavior, it usually indicates that something is very wrong, either medically or psychologically.
It’s always important to check out the possibility of an undiagnosed medical problem so the first thing to do is to make appointment with the pediatrician. Another possibility is that she has been hurt by someone and is scared to tell you about it or doesn’t know how. You know she hasn’t been abused or neglected by you but are you sure that she hasn’t been hurt by someone else?
Rather than scold, correct or punish your daughter, I think you need to have a quiet and heartfelt talk with her. Explain that you are very, very worried about the change in the way she is acting. Tell her that sometimes when people don’t know what to say, they act out their problem. Is she angry about something? Afraid? Sad?
One useful question to ask is this: “How would things be different if you couldn’t have a tantrum?” Sometimes the answer to that question gives us insight into what is bothering a kid.
The key in this is to be as calm, caring, and supportive as you know how to be — no matter what she says. She is only 6. You are 35. You can keep your head even if she can’t keep hers. If she is negative, simply stay with her and ask what else she has to say. Reassure her that you love her and that you want to help.
Once you have more information, you may be able to figure out how to be more helpful to her. If not, I suggest you find a family therapist. You and her father, if he is in the picture, need to learn new ways to support, encourage, and help your daughter. It’s important to lay down a good foundation now so that the rest of her childhood and teen years are not filled with strife and stress.
I wish you well.