toddler intentionally/self inflicting pain on herself

My 2 yr old toddler currently sleeps in her crib at night.  Occasionally she will wake up and start crying out loud for Mommy or Daddy.  We usually will let her "cry it out" in hopes she doesn't get dependent on us coming into her room to soothe her or bring her into our room for the rest of the night.  Recently we discovered (via a monitor/video camera) she would intentionally hit her mouth/lower lip area directly into the crib rail during her bouts of crying.  After this she would continue to cry and yell "owie, owie".  This behavior shocked us and we're worried that there may be something wrong with her.  Plus we don't want her to seriously hurt herself.  She is a normal 2 yr old toddler.  Good speech and motor skills.  Interacts well w/family.  We have a 9 month old baby in the house and she seems to have adjusted to his presence.  She doesn't do any of this odd behavior during the day.  Only at night when she cries for us.  Please advise or give us suggesitons on how to handle this.  Thank you all for your help.