What Causes Periods Twice In A Month?

Hi I'm 20 years old. I started my normal period on the 7th of february it lasted 4 to 5 days exactly like normal. Then two weeks later on the 21st I started my period again. This time its very light and very pink. It has lasted 13days now. I am still bleeding very lightly. I took a Hpt a lil over a week ago. It came back negative. My periods usually come on time, everytime. N only last 4 to 5 days. Please tell me wrong with me?