There is a possiblity for Bactrim (the antibiotic you are taking) to cause heart palpitations. If you are having heart palpitations accompanied by chest pain, it would probably be a good idea to call your doctor.
"Stomach issues", like acid reflux issues? If that is the case, I'd consider that problem more than the treatment you're taking for it. There is a link between stomach issues like GERD, and arrhythmia.