You didn't say what color the discharge is? Blood? Yellow? Whitish?
I would keep the panties that are stained... to show the Doctor.
My daughter had that once, but she was 5. It was just a slight discolored discharge... and it turned out she had a UTI. Which the Doc said would go away on its own...but to stop bubble baths, soaps in that area, and in fact the Doc suggested SHOWERS not baths and what not. It did go away. My daughter though, had no discomfort. No other symptoms, just the discharge.
If your Ped does not call back... call them back again. THEN, make an appointment instead of just making it an "I have a question" type phone call. If you want an appointment....they may be more likely to call back.
All the best,