You should go to the doctor or Nurse Practitioner. Pain often means soemthing could be wrong, as Gilda said -- a cyst, etc.
Hope you get this figured out and that it's no big deal, but better to be on the safe side here. You did not say how old you are -- my periods could fluctuate throughout my history with cramps/no cramps, etc. -- so maybe this is just some big hormonal shift -- but do get it checked out!