Now the reason I was asking more about the breathing is because his rate should be closer to ~30bpm at rest. So, if he is down at that rate now, I'd just keep an eye on that. As well, do monitor the gum color, since paling can be a sign of anemia (low red blood cell volume). So, if that rate stays elevated or gets any more pale, we'd want that checked. But he is allowed to be breathe quicker is he is playing or being active.
Otherwise, we need to consider this lump. Now 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 mass that has suddenly appeared and since he is so young, we'd be less worried about issues like nasty cancers, cysts, and benign growths. Instead, we’d need to consider trauma induced hematomas (blood blister like lesions), soft tissue swelling, abscesses, vaccine reactions, 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, to rule out allergic reactions and vaccine reactions, I would note that we can try him with a low dose of an antihistamine. Commonly we will use Benadryl (Diphenhydramine). A low dose (ie. 0.25mg 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 little 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 mass doesn't settle with antihistamine treatment then it does rule allergic reactions to insects or a vaccine sensitivity. And in that case leaves us with infection, traumatic damage, and more sinister issues. In that case, the best way to approach an abnormal mass like this is to have your vet evaluate the mass via fine needle aspiration (FNA). This is where the vet uses a needle to harvest cells from the mass. If the 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/cat-safe anti-inflammatories can be used to settle the swelling. Otherwise, if the above are not found, then the cells they harvest can be stained and the identity of the nature of the mass can be determined and whether it is something that is concerning or needs more serious treatment.
Overall, if this mass 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 or those breathing issues remain; then we’d want to have your vet examine him and sample this mass to diagnose each and initiate appropriate treatment to address it for Dave.
I hope this information is helpful.
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All the best,
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