Why is my girlfriend being verbally abusive in front of others suddenly. Acting like a Bully.?

I was going to purpose to my girlfriend of 4 years, last year, even found a ring, but suddenly she started to become condescending towards me, and is criticizing me more than ever. We're both in our 30's, working class people. Whats so bad about this, is it seems like she has started a new habit of doing this in front of her friends, and her family. She says these condescending things that are so hurtful. It feels like she's trying to show dominance. For Example, we were having dinner with her Sister and her sisters husband last weekend. Out of nowhere, she comes out and says in a nasty, parent to a child tone of voice " You always chew with your mouth open and its really rude, I've never told you this before but I notice it every-time you eat" I was like??? For one I don't chew with my mouth open, my parents used to beat me for that, they were very strict about table manners ( Catholic ) and when we're alone and I ask her what was up with her talking to me like that, say For example the situation with her sister that happened recently, she will just act like I'm crazy, when I tell her it's not right for her to talk that way in front of other people. She just carelessly sits there, and will give me some sarcastic " i'm sorry I hurt your feelings ' followed by telling me I'm " too sensitive" I don't know what's going on with her, I'm not overly sensitive, I was in the marine core, and was a Cleveland Police officer for 4 years!!! I have very thick skin. I don't know what her motive is for this why is she doing this. Even her sisters expression was sort of shocking, and it's extremely embarrassing.