Why was my martini so bad? What did I do wrong?

Last week I ordered a dirty Grey Goose martini (my favorite drink) and it was sooo good! I was in heaven! Last night I realized that I have everything at home to make one on my own!! Yay! So, I swirl some vermouth in my glass, fill my shaker with ice and vodka(GG), shake, pour into glass, add a little olive juice and a few olives! I sat down and took a big swig and ….YUCK!! It didn’t taste anything like what I had at the restaurant! It tasted like straight up alcohol! Blah!! What did I do that could make it taste so different?!?! Am I missing something? Help! :(