Hello, I am afraid that the expert you have requested is not currently available. Still I am Dr. B, a licensed veterinarian and I would like to help you with your wee one today.
How long has the lump been present?
Is it hard, soft, firm, or fluidy?
If you press on it, does it reduce in size or pop back into the body?
You noted it was under the rib cage, is it on the belly where you'd expect a belly button?
Can you take a photo of this? If you can do so and post them online, it will let me see what you are seeing. To post them, you can either use the paper clip on the tool bar. Or if you cannot see that on your phone/computer, then you can post them on any site (ie Flickr, Photobucket, Imgr etc) and paste the web address here for me to have a peek.