Yes the low potassium levels exacerbate irritability and anxiety. In several studies with people diagnosed with depression, increasing their intake of high potassium foods showed notable improvement in their day to day mood. Because potassium is used in neural transmission, increasing potassium intake in your diet may help improve mood and alleviate anxiety.
Yes it could be lower than normal potassium (or other salts) evels like Aicha says. However there could be a host of psychological or neurological reasons as well. It would be suggested (since the physician did not find anything physical) that you see a psychologist and/or psychiatrist, possibly even a neurologist. A physician does not typically look into these areas... Then again she could just be a highly emotional little girl. They can be that way, you know.... Lol. Good luck.