why has alcohol suddenly stopped working?

so for a bit of background, up until recently i was a bit of a lightweight, but for the past 4ish months, whenever i drink i feel absolutely nothing. i went out for drinks with a friend tonight and had around 10-15 standard drinks, but there was no effect, i didn’t even feel slightly tipsy. i don’t drink very often, maybe once a month, or once a fortnight at most, so i don’t think it would be a tolerance thing. does anyone know what might cause this?