Blackout and switching personality when drinking suddently

the last 2 times I’ve been drinking I’ve completely lost control of myself, I start to be rude and mean to those around me for example my girlfriend, I become a completely different person, and the day after I remember almost nothing. the same thing happened both times when I’ve almost finished my drink. both times (8 0.5 beers). how has this happened now suddenly when it has never happened before. What can I do to prevent this from happening?