The question of explaining why you are not drinking is a very tough one.
I have asked this question many times on SR and you can see from looking at my dates that all of my attempts have been shams. Here are some of what I have tried:
I'm watching my weight.
I'm the designated driver.
I'm on medication.
I'm not feeling well.
I have a big meeting tomorrow.
I'm broke.
None of them addresses the question. AND one important reason I have used false reasons is because I didn't believe I would stay sober, so I didn't want people to know I am an alcoholic.
I cover that up by saying that I don't want them to feel sorry for me or funny around me.
I will never be okay until I say when asked, by one of my daughters, "Why aren't you drinking?" ... "Because I shouldn't drink. I shouldn't drink because I can't handle it well. And that's because I'm an alcoholic."
I hope you get this figured out now while you are young and have a bright future and a girlfriend who loves you.