Why didn’t I get/feel drunk?

About a month ago I went out with some work colleagues because one of them was transferring to another store. While out, I had six drinks (and a shot of something red?), with four of the drinks being vodka coke, and at least one of the others having rum in it. I drank these drinks over a period of about four hours.

This was only the 5th time I have ever had any kind of alcohol.

Also, before I went out I only ate half a McDonalds Big Mac and some fries.