It is really odd for a lump that large to appear at that size in only two days....so, either it has been there for awhile and was slowly getting bigger and was now just first noticed.....or, it is something called a hematoma or abscess. A hematoma is an accumulation of blood under the skin that occurs when an animal bangs themselves on something by falling, or slamming into something (or something or someone bangs them) and a capillary bursts under the skin allowing some bleeding. If this is the case, then this will get progressively smaller over the next couple of weeks and go away on its own.
An abscess is a pocket of infection trapped under the skin. This is usually acquired from a bite from another animal or some other puncture type wound. The vet would diagnose this by putting a needle in to see if it is filled with pus. This type of problem requires opening of the abscess, draining, cleaning out, and then oral antibiotics.
If it isn't either of the first two, then unfortunately, there is no way to know what a lump is definitively, unless it is removed surgically and sent in for biopsy. One way to try to find out is by doing a test called a fine needle aspirate. During this test, the vet places a needle inside the mass and pulls out some of the cells and other materials inside. This is then sprayed and smeared on a glass slide and is stained for viewing under a microscope. A pathologist can tell what a lump is 50% of the time with this test. The major benefit of this test is that it may give you an answer without having to put your dog under anesthesia.
Most people opt to surgically remove the lumps, even when benign, because this will give a definitive diagnosis, and the lump will then be gone if it is malignant (cancerous). Furthermore, many benign lumps do continue to get bigger and this can be cumbersome, unsightly, and sometimes uncomfortable for the dog.
I hope that this information is of help to you, and I wish you the best of luck with your dog.