What Causes Irregular Periods?

Last month, I was late for my period by 10 days. Which is not really normal for me. Then after I finished a normal period, I bled again 5 days later for 3 days. Not just spotting, but steady bleeding. Then it stopped. Now I am late AGAIN by a week now, and I took a pregnancy test and It said it was negative. What is wrong with my body and is this something serious that I need to be concerned about?