Hello,
This may be a hematoma (a bloot clot) or a seroma (a ruptured blood vessel with secondary inflammatory changes that looks like a cyst full of blood liquid-like fluid).
It still could be a cyst.
Now, if it has ruptured, the chances for it getting infected are substantially higher, and it could even form an abscess.

Therefore, regardless of which of the above it could be, definitely, worth a veterinary checkup.

Your vet may recommend a fine-needle aspirate test depending on what it looks like and if it is necessary to more accurately diagnose it.

A fine-needle aspirate test is performed usually without anesthesia and is very quick. Your veterinarian takes a few needle samples from the lump (the needle is no larger than the needle used to give vaccines). The sample is smeared on to microscope slides and submitted to the laboratory. It is not always as definitive as a biopsy (this is a full tissue sample), but often it can yield enough information to determine if it is anything to be worried.
http://www.petplace.com/dogs/cytology-in-dogs/page1.aspx
http://www.petplace.com/dogs/fine-needle-aspiration/page1.aspx

Hope that helps
Good Luck
I wish the best for both you and your pet. Please let me know if I can be of any further assistance.
I truly appreciate positive feedback if my advice has been helpful or informative.
Dr. Andy