When people are turned on, we're a lot more likely to register something like sucking on breasts as pleasure instead of pain. That is why your girlfriend is asking for you to be more forceful during foreplay.
However, once we aren't as turned on, it can feel a lot more like pain. Something that might have felt nice before is suddenly much more sensitive and can possibly hurt.
As for her health, it probably won't do any damage. Obviously, if you are doing something hard enough to draw blood or leave bruises, something is wrong and yes, you could be hurting her. But provided you aren't, it shouldn't have a long term affect. There may be sensitivity for a couple days at most, but that's about it.