Had a annual PAP Smear on 07/14/15. Results came back for all STD's negative. My tubes have been tied for 2 years. My period blood sometimes has a rotten smell. What can be causing the rotten smell? I never smell my Vagina smelling rotten only the Period

Bacterial vaginosis. Very common around cycle to have a change in bacterial growth in the vagina with the "good" bacteria dying off to be replaced by "bad" bacteria which leads to a bad odor. It can persist after the cycle. It relates to changes in the vaginal acid/base status or pH. Symptoms often onclude itching or burning. It is not an STD. It is relatively easy to diagnose and treat if confirmed. .