I found a lump on my new kittens ribcage Any encouraging stories?

I just got this kitten 3 days ago from the humane society shes spayed and just got a so called clean bill of health. we were snuggling and I found a lump on her side in between her ribcage its movable. dose anyone have any encouraging news to tell me. I'm taking her to the vet asap luckily she came with a free month of kitty insurance