Most likely it is a cyst or a pimple. Tight clothing can cause decreased air flow to that area making it more prone to a cyst or a pimple. Yeast infections also cause small lumps, but they will itch.
It could be an allergic reaction. You could be having symptoms from changing soaps, detergents,condoms or spermicides. The best thing you can do is talk to your doctor.
Lumps sometimes come up on the vaginal opening but it is not always. It is due to the reason of using unhygenic innerwears and not proper washing of the opening.
