I am sorry that your baby is suffering from loose stools. Your baby has started passing loose stools only recently, but she has been taking formula milk since a long time and was passing normal stools before. So definitely its not because of non introduction of solid food items. As suggested by doctor, it could be due to bugs like viruses. Make sure that feeding bottles, utensils you use to prepare formula are clean and boiled well. Sometimes it could be just a normal variation which gets alright by its own.
So just keep giving her adequate formula milk, maintain hydration and she will become alright soon. If loose stools persist or condition worsens, a pediatrician
opinion may be sought. Take care.
Dr C. Balaji