Now that just about sucks... Brace yourself as best you can, guy - in my experience, that shit moves FAST once it gets going.
I worked for some folks who had a dane bitch that went that way, and at about the same age - Discovered a teat-lump roughly the size of your typical marble, vet carved it out, and all seemed as well as could be expected as they waited for the biopsy results. Malignant, of course. Followup visit at 6 weeks, new lump about the size of a pea in another teat, and at least a dozen head-of-a-pin sized bumps (you really had to hunt for them, and when you found one, the first thought was "might be a tick?" - we were in terrain where it was basically impossible to go outside and NOT come back in with at least 2 or 3 of the little bloodsuckers on you) randomly scattered, just under the skin. Doesn't look good - Shoot some X-rays. The pics of her lungs had so many spots it looked like she'd been hit with half a dozen loads of buckshot from close range. Too many tumors in too many places to even attempt surgery. A month later, she had marble-sized bumps everywhere the pin-head sized tumors were spotted, and several the size of golf balls in assorted other places. About another month after that, they decided to pull the plug when she went off her chow and started hacking up blood. At least it was fast... That's the only good thing that can be said of it...
Good luck, guy...