Having periods during pregnancy is a myth. There are women who have bleeding during pregnancy, but it is not a period - having a period is the shedding of the uterine lining. This would take any embryo that was implanted there along with it, thus ending the pregnancy.
If you are having bleeding and had a negative test, it is highly unlikely that you're pregnant. I would make an appointment and have some hormonal tests done to make sure that everything's OK.
Good luck!
Rachel Inbar
www.FertilityStories.com