What does it mean when a girl says its getting too serious and she needs a breather?

I would say under very few circumstances does that mean anything good. Sure there are some women who are very busy with their careers, maybe need to find themselves, have children, or just got out of a relationship and want to get some balance in their lives before moving forward full steem ahead. It does happen and it is understandable, however I would say majority of the time it's a nice way of saying she's not happy in the relationship and wants to breakup.