What Causes Bad Odor In The Vagina?

Hi, i am a little nervous to ask about this. I just started birth control today because my boyfriend and I are pretty serious. We have infact had unprotected sex about four times so i decided to take a precaution for safety which i should have done before. My problem is simple. Every now and then my vagina will give off this wierd smell, it doens't itch in anyway at all or anything like that, but i am nervous because i am not a doctor and I don't know if i should be concerned about this or not. Help ?