What if this is actually a good chance to show what you can do? In my experience, bosses often give extra work to people they trust to handle it. Saying no right away might make you seem like you can't adapt or aren't a team player. I got my biggest raise after I quietly took on a huge, unexpected project and nailed it. Sometimes you just have to dig deep for a bit, prove you can manage the load, and it pays off later. It's stressful, but it might be a test that leads to better opportunities.
That "test" never ends though. They just keep moving the finish line. Saw it happen to my friend, kept getting promised a promotion if she handled the extra load. Two years later, same job, way more work. It's just a way to get more for less.