I have had a normal period for 6 months straight same day every month.  Suddenly I started spotting every day!  Some days it's super heavy and some days are light.  I'm not on birth control and have taken a few pregnancy tests and STI tests and stopped having sex completely.  Tests are all negative.  What could be the problem?

Abnormal vaginal bleeding can be caused by a variety of reasons.  The most common cause of abnormal vaginal bleeding is anovulatory bleeding, which is unpredictable endometrial bleeding of variable flow and duration.  It can be caused by a change in weight, an eating disorder, intense exercise, stress, illness and medications.  You can also have abnormal vaginal bleeding due to a pregnancy or an infection, such as a sexually transmitted infection, so it was prudent to rule these out. 

If this has been going on for more than a week or two, make an appointment with your health care provider to evaluate the bleeding, to rule out illness, such as thyroid problems, and anemia, and to treat the problem.