D.S.
Have her checked for a yeast infection. Yeast can come from antibiotics and if she has been on antibiotics for a while she could have one. It can give off a fishy smell and can also be very itchy. Do you notice any type of white discharge in her vaginal area? In the meantime try to get her to eat some yogurt that can help with yeast. If it is yeast she will need to be treated so call the doctor immediately. Yeast will also grow in warm moist areas so change her or if possible keep the diaper off her as much as you can so the area can stay dry. Good luck and keep us posted!