Just noticed a lump on my dogs left rib cage?

My dog was spayed about 5 days ago. Last night I noticed a lump about middle ways of her left rib cage. Im not sure where any shots were given but I thought that could be what its from. Its about as big around as a quarter and is a little squishy and can be slightly moved. I take her in 6 more days to get her staples out and I planned to ask about it then. Could this something serius that I should take her asap?