Did you use a condom because if not STD can also be a possibility. Sweety you cant think some one is clean just by looking at them. They have kits at Walgreens and CVS you can check to see if its bacterial or an infection its called Vagisil Screening Kit
You probably are getting a yeast or bacterial infection. If you start having a lot of abnormal discharge, itching, burning, and/or swelling then that's almost definitely where the smell is coming from. Yeast and other vaginal infections are actually pretty common so there is no need to be embarrassed or hide it from your mom. Although sex can increase the likelihood of an infection (especially if your partner isn't practicing good hygiene) anyone can get an infection without sex being in the picture.
Yeast infections can be treated with an over the counter treatment. If it's bacterial though you will need to have a prescription from a doctor. And if you're unsure about which it is talk to your doctor about it.
Again nothing to be embarrassed about- and you definitely tell your mom about it without her jumping to sex. Although, fair warning, she might ask.