You may want to try an antifungal cream. Before applying it, take a warm bath and make sure you dry the area well. If you can, let the area be exposed to air for a bit as well. This can happen as the weather gets warmer. Avoid nylon panties or pantyhose. Stick with cottons or cotton blends.
The other thing is perhaps it is an allergic reaction if you have used a new soap or cream or detergent.