Why do I suddenly have two periods in one month?

I normally have a 28 days between my periods-like clockwork. But last month I started my period early. It lasted for about 5 days then ended. About 3 days later it started again, but it was very light, I just wore a light tampon. I thought it was weird, I've never had 2 periods in one month. I decided to wait for the next month. Now, the same thing happend. Can anyone explain this to me?