5 month old leonberger with a soft, squishy, movable lump?

I have a 5 month old leonberger puppy. She has a soft squishy movable lump on her side, (by her ribcage). I just noticed it today, as everyday we have our daily grooming sessions. Just wondering if anyone would have any idea as to what this could be? She is going to the vets on wednesday and I will raise the concern then but in the mean time I figured I would just see what everyone else's opinion would be