Do ulcers always bleed?

Of course... Did you ever have canker sores in your mouth? Well UC ulcers are somewhat similar... They bleed when they're irritated, but even when they don't they can hurt like hell. Also, consider that you can have occult bleeding, i.e. blood is not visible in the stool, but it's still there (this happens because blood oxidizes and changes color from red to brown or black, so it becomes indistinguishable from stool on visual inspection).