My dog has a burst lump on her chest. Is it cancer or an infection?

Unfortunately, even in person I would be hard pressed to give you a diagnosis based only on appearance. When a client comes in with a pet that has an unknown mass, we typically consider the following as possibilities, taking many factors into account: a mass/tumor, an abscess, a cystic structure, an insect bite, an allergic reaction, or a bruise from trauma. If the structure has burst open, it is most likely not allergic or from an insect. Depending upon your dog's age, the time the lump has existed, the consistency of the fluid you have noted, and the presence / absence of any other clinical signs, one of the other options is most likely. Something the vet might be able to do is obtain some cellular material from the affected area, place it under a microscope, and see if there is any cellular material that is useful in obtaining a diagnosis. Other times, appearance alone can at least tell a vet whether or not there is an active infection. As far as the possibility of cancer, tumors are certainly able to open up as you have described. They are typically more common in elderly dogs, though even this is largely dependent on the type of tumor. I would schedule a visit with your vet sometime soon. I would also in the meantime prevent Pancakes from bothering with the area. You can either buy an e-collar from a store / vet, or try and make something homemade until a commercial one can be obtained (with it being Easter, finding a store open that has one may prove difficult). The purpose of this is to prevent any current infection from worsening, or absent infection from beginning. Hope this makes sense.