What Causes Pelvic Pain At The End Of Menstrual Bleeding?

I have menses on 12th jan on 6th day of my cycle i feel pain on my left lower side pelvis bleeding almost stop , my cycle regular 28days ,cycle 2-3days before previous cycle i am trying to concieve 2nd time my last menses 24th december but this cycle come early now i feel pain off and on sharp ,what is this