Hello, I'm Dr. Deb and will do my best to help you today.I'm sorry for this concern for your dog but if the surgery just occurred today, then there are several possible explanations for the swelling you're seeing:1. The swelling could be secondary to blood; a small blood vessel may be leaking into the area but has most likely stopped or you'd see leakage or the swelling would continue.2. There could be leakage of serum into the space secondary to trauma of tissues during surgery.3. Dogs can experience suture reactions to the material used to close the incision but this usually takes several days to occur. Similarly, infection can take place but, again, wouldn't be seen right away.In some situations where cancer is diagnosed and if the entire mass isn't removed, recurrence at the incision site can take place but this wouldn't be seen on the same day as surgery either.#1 and #2 can happen when any surgery is done and is more (or less) likely depending where on the body lumps are removed.If your dog is otherwise acting normal, if the incision is not pulling apart, if there's no leakage or seepage, then I might apply a warm compress to the area 2 times a day for about 5-10 minutes.I hope this helps. DebIf the swelling doesn't appear to go down in size or enlarges, then a recheck may be prudent. If this were my case, I'd insert a needle into the swelling to determine if fluid is present or not.