It's even weirder that people are saying that she might be nervous laughing when in the same OP he states

I remember before we were even together she was telling me that I was "super cute," and I playfully said, "you might think so, but I don't agree." She smiled and said, "Shut up! You're cute because I said so, and I have great taste in guys, thank you." For the record, I don't normally talk about myself like this. I am not one of those people that is always reminding their partner that they are more attractive than them lol.

So its apparent that she is willing and able to stand up for her bf when he's around but all of a sudden when he's not around she becomes passive? Doesn't add up to me.

As for what OP should do, personally I think he should take some time to cool himself down (so he doesn't say anything bad due to emotions) and confront his GF on this (the calling him ugly and the not standing up for him). If OP's GF is a good gf she'd acknowledge that she was in the wrong and change her behavior to reflect it.