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.