i keep on blacking out

hi im a first year uni student so there are lots of drinking activities going on. i used to have a high tolerance but suddenly its like a switch has been flipped. i dont feel drunk after having maybe two drinks, so ill drink more and then suddenly ill black out. im so confused. i also dont completely black out i have some memory its just extremely patchy and i cant remember getting from place to place or meeting different people. for context i drink just under 35cl (a small bottle) when i go out (vodka). i also sometimes mix with redbull if that has an effect?