A "bump" can be just about anything from a small vaccine reaction (which should go away in a couple weeks if you had vaccinations given recently), a tumor (either cancerous or non-cancerous), an oil filled cyst/infected hair folicle (basically a doggie zit), an abcess (infection under the skin) a bruise, or any other number of things.

Only your vet will know what it is by feeling it and possibly doing x-rays or a biopsy.

If it is rapidly growing or bothering your dog, or draining pus/blood,then I'd see if you could get your dog in somewhere sooner. Otherwise just be patient and wait for your vet to diagnose it.

I hope it turns out to be nothing serious. Please keep us updated.