What Causes Menstrual Bleeding To Be Prolonged And Light?

Hi, i have a question.... on June 7th i started my period.. and it wasnt like a normal period, my bleeding was real light and a brownish color.. it lasted for 13 days... which ive never had a period that lasts that long!!! Then today which is 8 days later ive started the light bleeding again... im not sure wat the the deal is and its worrying me... me and my husband dont have health insurance at the time, but are working on it... We have had unprotected sex and i took a pregnancy test today and it was negative.... do you have any suggestions or answers that could possible help me to ease my mind??? Thank you....