Every time I drink recently I blackout even though I'm not getting overly drunk, any explanations?

I was once a heavy drinker, but lost a lot of weight in the last year and don't drink too often. Now every time I go to the bars I end up blacking out to the disbelief of my friends who insist I wasn't that drunk. this has never been an issue for me but I'm afraid something bad could happen if this continues. Is there something that could have changed that is making me abnormally prone to blackouts without binge drinking?