Ummm. If you were going to get nute burn from the medium, I would suspect it showed up during the seedling stage, as this is the most sensitive time.
Have you started feeding?
Is the problem progressing?
Do you PH your water/feed?
Do you have a scope to rule out bugs?
I could venture a guess as to what's going on but I don't want to steer you wrong. This doesn't look to be anything I've encountered.
You may want to send her to the infirmary and have the experts take a look.