Something's wrong with your face...
Naw, just messing.... But seems to be something wrong with your screen or the light detecting unit on your phone. When the phone sees no light then the screen automatically shuts off on a phone call, and when you put the phone away from your ear it detects light and will then turn the screen back on. You should first make sure if its the way you angle your phone, if you notice that there is no way that light is going to the phone, call apple support and they may fix it or swap it out with another phone. This is of course if you haven't dropped your phone or anything.