"No thank you. Why do you ask?"
Sure, it might be that your team is short-staffed, or that they are trying to squeeze more work out of you. But it could also be that your boss is trying to set you up for future promotions: if you are in a male-dominated area, you are already at a disadvantage in the promotion department; and if your male co-workers are all working longer hours than you are, you will have almost no shot at the next rung up the ladder. So if your boss is trying to promote you in particular, or women in general, she might be suggesting the additional time to help you move up the ladder, so you don't face the "gee, I'm not sure she's really as 'committed' to the job as Dave or Bob [who are routinely at their desks at all hours]" death knell.
If that is the case, then she needs to hear directly from you that you are not actually interested in additional promotions if those come at the cost of additional work, and that you would instead prefer to stay where you are now under the terms of the deal you negotiated. Otherwise, she may get angry that she is doing all this stuff to try to help you, and you are ignoring her advice and not even appreciating her efforts (which may involve some personal risk to her own career, given the male-dominated department).