Thanks guys. Jazzy is due for a check up in a couple of weeks. As long as it doesn't change in size or start to bother her I will show it to the vet when we go in.
It's certainly worth having a vet take a look at it, just to rule out anything serious. Chances are it's just a lipoma, the area around the ribcage is one of the most common locations to develop these fatty tumors. The fact that you can move it around and it's not hard is good, it means that it's not anchored to anything. Please let your vet diagnose it, though, it's the only safe thing to do.
Ghilly
I have a lumpy dog. She's always had some small lipomas and tiny benign cysts as long as I have owned her (7yrs) I watch always for any signs of growth or change in these lumps. I am very used to what lumps feel like, what they could signify, and what a suspicious lump feels like (my mother had breast cancer a while ago, so I got used to what sinister lumps felt like) But even though my dog only has benign lumps, which do not alter or grow, I got all of them checked out by a vet. I think for total peace of mind, that is the wisest thing to do.
That said, the lump you describe does sound like it could be possibly a small fluid-filled cyst, or it could be a small lipoma (benign, and consisting of fatty material, perfectly harmless, as long as it does not grow too big, interfere with surrounding tissues or muscle)
No doubt the vet will tell you just what you have already suggested. Keep an eye on it, to make sure it doesn't grow, change, or become painful. BUT, Like I said, I would ALWAYS check out any lump straight away with a good vet. It's better to be safe, and pay an examination fee, than be sorry because you made a mistake.