Hello & welcome, I am Dr. B, a licensed veterinarian and I would like to help you today.
I do apologize that my colleagues could not aid you sooner but I wanted to touch base with you about Biff. As I am sure you can appreciate when we see any lump on our pets, we do have to consider a range of issues. Still if this is a lump that has suddenly appeared, then issues like nasty cancers, cysts, and benign growths would be less likely. Instead, we’d need to consider abscesses (eg tooth root based), trauma induced hematomas (blood blister like lesions), soft tissue swelling, and insect sting induced allergic reactions.
Now with these in mind, we can start some supportive care to try and rule out some of these. To start, if there is any chance of a bee/spider/wasp sting or bite, then we can reduce allergic type swelling using antihistamines. Commonly we will use Benadryl (Diphenhydramine; More Info/Dose). A low dose (ie. 0.5mg per pound of body weight twice daily) is often enough to reduce these signs over a few days. We do usually like to keep the dose low, as they can have drowsiness with this medication (just like people). As well, of course, this medication shouldn't be used if your wee one has any pre-existing conditions or is on any other medication without speaking to your vet first.
Furthermore, to reduce swelling with any of these sudden appearing concerns, you can also start warm compressing this lump. This can reduce inflammation as well as encourage hematomas and allergic reactions to settle. Just to note, you can make a safe warmer for use as a warm compress by filling a clean sock 2/3rds full with uncooked white rice. Tie it closed and microwave (approx 1-1.5 min). Before use, do make sure to shake to allow the heat to distribute before using as a compress. (If it cools, you can re-warm as required).
Now if you use the above, but the lump doesn't settle with our supportive care, then we'd need to think about those other issues. In that case, the best way to approach an abnormal lump like this is to have your vet evaluate the lump via fine needle aspiration (FNA). This is where the vet uses a needle to harvest cells from the lump. If they remove pus, then this tells us that there is infection present and antibiotics can be dispensed. If blood or blood stained fluid is removed, then trauma was most likely and pain relief/ anti-inflammatories can be used to settle the swelling. Otherwise, if the above are not found, then the cells they harvest can be stained to tell us what is present and whether it is something that needs more serious treatment.
Overall, if this lump is sudden in appearance, we would consider those initial sudden onset concerns. Therefore, as long as it’s not painful or obviously draining pus, then you can try the above to rule out those aforementioned concerns. Otherwise, if this doesn’t settle; then we’d want to have Biff's vet sample this lump to identify its cause so that appropriate treatment can be initiated to address it for your wee one.