My girlfriend talks in her sleep?

So sometimes wen im talking on tge phone with my girlfriend she'll doze off and begin to sleep talk . When she does this shes always sayn how her ex boyfriend is better than me in such and such ways. When i tell her what she told me she says she has no recolexion of what happened. She cries and swares its not true. Why do you think she says these things that she says arent true? Please help