Alcohol no longer getting me drunk

I’m a 48yo f, I have never been a big drinker but like to get a bit drunk on the weekend. Usually I have 6-12 units once every 2 weeks. This will get me in a good mood and will have minimal hangover. My problem is that the last 2 times I have had a drink it has had no effect. So last night I went for it. I had 23 units over a 4 hour period, I was only mildly tipsy by the end. This has got me confused. Would there be any physiological reason why I can no longer get drunk