How to tell if a 4 yr old is telling the truth or making up stuff?

I have a 4 yr old in PreK. The past two weeks she has not be her normal self at school. She has been very defiant. More then usual and has arose to extreme negative behavior. I know my child is no angel. She is very emotional child. But the last two weeks, she will flat out refuse to step foot in class. With arms crossed. She is rolling on the tables and floor. Will not listen to the teacher at all. She has hit the teacher at least three times in the past two weeks. They sent her to ISS, which is for older kids. My husband got her and she comes running out with ice cream. Her tv was taken out of her room previously, and all toys locked up. She sat in her room all day. So when I came home from work, ask her why she got sent to ISS, she told me two particular kids keep pinching her and one hit her in the face.I did record the conversation. This is the second time she been telling me someone is picking on her. I told them the first time and they said they will look in to it.I did point out if something was going on at home, she would be acting out before coming home from school. But this behavior is starting before class and lunch room. She said she likes her teachers and her class. I said something in the class dynamics has changed. Not sure what, but for her to act out to extreme behavior has me truly concerned. Any Insight would be helpful...