Your panic attack may have been brought on by mixing the alcohol with the Zoloft. Alcohol is a depressant, which temporarily numbs your senses, but if you are taking anti-anxiety medication, the two can conflict. I'm sure there is a warning against alcohol in the documentation that comes with your meds, if not a sticker on the bottle itself. You may have an irritable stomach, since mediations can cause irritation. Adding alcohol is just like pouring gas on a fire. That probably was why your stomach hurt and caused you to panic. That obviously kicked in the heart palpitations, and ta-dah, panic attack. Adding the Inderal to the mix could have been dangerous also.
Concentrate on your life, not whether or not you are likely to have panic attack. Just the desire to quit your meds could have brought on the attack. Your subconscious could have been focusing on what would happen to you if you quit your meds and had an attack, without something to back you up. Just a thought.
Let us know how you are doing.