What Causes Having Two Periods In A Month?

Hi, I just wanted to know if having two periods in one month is normal? I've been going at the very beginning of each month for a while, this past time i started on May 31 and it went to June 3..and now I've started again on June 25, and I'm not due to have a period for another week, and I took a pregnancy test, and it was Negative..