Hello & welcome, I am Dr. B, a licensed veterinarian and I would like to help you with your wee one today.
Now as I am sure you can appreciate, we can see lumps arise in our dogs for a range of reasons. Now the location is a worry since this is a common site for mammary masses (where 50% are cancerous). But given that this has appeared over 2 days, we'd be more suspicious of a soft tissue swelling, hematoma (blood blister), abscess, cyst, or insect induced allergic reactions. As well, if she is not spayed and there is any chance she has milk (you can try to express this), then a false pregnancy or mastitis could appear this way.
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 or 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 @ http://www.petplace.com/article/drug-library/library/over-the-counter/diphenhydramine-benadryl). 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 she 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. 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/pet safe anti-inflammatories can be used to settle the swelling. If the fluid is clear, then a cyst is likely and that can be drained or removed. 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 has been so 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 may have been present for longer then a day; then we’d want to have your vet sample this mass to identify its cause so that appropriate treatment can be initiated to address it for her.
If you have any other questions, please ask me – I’ll be happy to respond. Please do rate my service once you have all the information you need as this is the only way I am credited for assisting you today. Thank you! : )