I would say it is a combination of both the job and alcohol.
Obviously alcohol, whether it be what you were drinking or just alcohol in general, does not agree with you and turns you into the angry/disagreeable drunk. Alcohol affects different people in different ways and we must work out for ourselves which we are. Some people are happy drunks and some are not.
I think the stresses of the job don't help and it may be that you are not dealing with them as well as you think. Only you will know the answer to that one.
What is best to do is to ask yourself some questions and answer yourself very honestly. You don't need to tell anyone else your answers, it is no one else's business, but be honest with yourself or it could be detrimental to yourself.
Good luck. X