My dog has a lump by his rib cage?

I think you should go to your vet because my dog had a similar bump by her rib cage and in the beginning it wasn't painful for her at all. When i took her to the vet, they drained it and tested the fluid for cancer because when they saw it, that could've been a possibility. Luckily it turned out to not be cancer however the bump keeps returning and now its starting to bother her so they will be doing more testing. My advice is to take her to the vet and let them test the fluid just to be safe and it doesn't turn out to be something painful. Good luck. I love dogs!