I feel this may have been something you needed to have discussed in negotiations - a clear set of guidelines as to how you communicate such feelings to each other. It could be, for example, that you agree that you cannot refuse any task so long as it does not break a hard limit but that you are allowed to express how much you did not like it during a defined afterplay moment. You could also agree that you can refuse a task but your Master must them assign a punishment for not completing that task which must be something you will do but is in some way as bad as or worse than the original task you refused. Maybe, for example, not completing the task will lead to a set period of cum restriction or a certain number of hits with a whip or sitting in the corner balancing two books on your hands. Something you will do but may not like too much. If these things are set in advance then situaitons like this are easier because you, as teh sub, know your boundaries. You know that you can refuse and what the penalty is or that you cannot refuse but you will be able to talk about it later.

You may also have got away with trying to negotiate there and then. Maybe suggesting the small steps and then offering an extra task that is punishment for you or pleasure for him. This, of course, would always depend on the exact dynamic between you and your Master as no relationship is the same.