If he doesn't use a condom with you, why would he use a condom with other girls? A lot of men don't like condoms because it dulls the sensation. Did he TELL you he had been checked and was clean, or did he SHOW you the PAPERWORK from the DOCTOR'S OFFICE? You can't always believe everything you're told. You have to use some common sense and protect yourself too. You may care for him and not want to "rock the boat," but he should also care for you and not mind your probing questions.
Make an appointment with a doctor and get it checked out. Do not try to self-diagnose, as there can be so many causes for an unusual smell. It could be an STD, but it could also be a yeast or BV infection (both non-STD infections) which sometimes arise from having sex or going on the pill. If you have a disease (which sounds unlikely in your case), then you're next step will be to get treatment. Get it checked out ASAP, as some infection if left untreated can lead to permanent damage. Being a sexually active woman adds on a whole new layer of complicated responsibilities, so make sure you take care of yourself if you want to have a long, happy sex life. Good luck!