Is it soft or hard? I'd definitely have a vet check it out...Sandie had a soft lump on her side recently and had to have it removed by the vet, then sent to the lab. It turned out, thankfully, to just be a hyper sensitive reaction to a bug bite - but the vet was talking things like cancer or fatty tumors and that's why she wanted to check it out.
So it could be nothing more than a reaction to an insect bite (yes, even this time of year...I live in Ohio where all the bugs have been dead since October, yet Sandie still manages to meet up with possibly the ONLY living spider around and have a reaction to it!!). But it's better safe than sorry with these things, so I'd call the vet to see what he/she thinks.
"The times when you have seen only one set of footprints in the sand, is when I carried you..."