I had the same reaction as you. Remember there is usually a temporary increase in anxiety when a dosage is increased. The increased anxiety can cause more stomach acid and therefore your symptoms. I confess I take omeprazole for the heartburn. The burping went away with the anxiety. My stomach burned so bad I thought I had an ulcer. You can get prevacid or prilosec over the counter. Either one should help. Good luck and excuse yourself!
I've just gone up to 25 mg of Zoloft, and my stomach is a mess also. It should be a temporary side effect that goes away as your body adjusts to the medication. I take Pepto when it gets bad. It's unpleasant, but I prefer to go through this temporary discomfort than live with the awful anxiety!