It could be infection, it could be hygiene issues, it could be a lack of fresh dry air to the vaginal area.
Vaginal ph (acid level) may also effect the odor slightly. Recent unprotected sex
will also cause an odor. I recommend no underwear to bed when not on your period, shower over tub, cotton panties over thongs and non cotton, and wiping front to back as a start. See you gyn if odor is new.