What can I do about mood swings?

I’ve been having mood swings lately. My girlfriend gets mad at me because one minute I’m being nice and sweet towards her and the next minute I start yelling at her. It’s for no apparent reason, I don’t mean to but it just happens. I’m stressed from work because I’m leading a presentation for possible investors. I’m not sure if the stress is getting to me or what. I was never like this with my girlfriend before. What can I do about this?