bleeding and smell

Since June 2005 on the 3rd of each month i get very dark but light bleeding, it also has a bad smell. The bleeding normally only lasts a few hours in the morning then is gone for the rest of the day,,this goes on until the 10th of each month when i get my regular period which is completly normal,after my 4-5 day period is done ,there is no smell or dark bleeding, I went to a dr and was told i had a bacterial infection and was put on medicine as well as vaginal inserts,i also had apregnancy test done. I am negative on that. So i fifnished the meds,and went back the dr said all is gone and the smell was gone...But the very next month everything came back? anyone have any idea hat this is?