Cancer is often painful in and of itself, so that is also something to consider.
We know that blood free in the abdomen is irritating, so it probably is uncomfortable and possibly painful to some degree. However, it is difficult to quantify how painful it is, since dogs with internal bleeding usually are lethargic and weak as well. This makes it difficult to assess if their behaviors
are due to pain or weakness.
In my experience, dogs with tumors
in the liver or spleen often have ongoing low to moderate levels of pain even when not bleeding. Occasinally some dogs are significantly painful. During a bleed, they do not seem to be in extreme pain, but are definitely more uncomfortable than they were before the bleed in addition to being weak.
You have my sympathies. Cancer is a tough foe to battle, and end of life and quality of life decisions are some of the hardest things we do for our pets. Good luck, and hang in there.