Liz Buchanan BVSc MRCVS
Keymaster
Hello! There are a few possibilities for raised lumps under the fur that appear out of nowhere. Insect bites, including flea bites, are sometimes accompanied by redness or pruritis (itchiness). Raised nodules that appear to be more nodule than itch could be urticaria which can vanish almost as quickly as it appears or can take a few days. Urticaria can be described as a kind of allergic reaction and is usually mild, but we would advise letting your vet know as some dogs have more severe attacks and there are other possibilities that they may want to rule out. Furthermore, once a dog has had urticaria once, they may do so again, so it's worth asking if there's anything else that your vet would want you to do or know.
Other, rarer differentials may include ringworm, wet dermatitis or multiple lipomas (much less likely to appear suddenly).
Best of luck.
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