Never had this happen before

My periods usually come every month....my cycle is between 32-40 days and last about 5-6 days long. I am not on birth control, my partner and i are kinda of trying, but what is weird is, my period came twice this month. My period started on May 9th- 13th then stopped and came back on May 20th-24th. Both seem to be a normal periods...why did this happen? Is this a concern i need to address and how can i calculate my fertile days... from my 1st period this month or start with my second period this month?