How to find the class name of a new image from the pre-trained model
In the original training session, there will have been the mapping between class name and numerical class ID (e.g. Cat = 0, Dog = 1, ...). The model you use will output just the IDs and you need to find the mapping that was used.

If your new data has been added in addition to the old classes, they should each have a new unique ID. If you used VGG architecture to train on your new classes from scratch, then the model will not be able to make predictions for the original VGG classes.