Sometimes a small amount of alcohol can cause you to black out. It's not always a 1:1 ratio. Sometimes I can drink 20 beers and be fine, sometimes I drink two and have the worst hangover ever. I've seen this happen to people before. And if you blacked out, you might have drank more than you thought. One thing to also consider is if you're taking any medication. Another thing to consider is that this stuff happens sometimes. I've been around drunk people for enough decades to know that sometimes people get disproportionately drunk compared to how many drinks they had, and many times think they were drugged but forget that alcohol itself is a very powerful drug with unpredictable results.