Gallbladder is on the right side, and the pain is typically on the right side. Really bad pain behind or just below the right breast. Women who have been through labor and delivery sometimes describe a gallbladder emergency as worse pain.
Ulcers typically occur in the stomach or esophagus, so that would be in the middle of your chest high up, or towards the left breast.
Obviously, we can’t know how much pain you are in or how upset your stomach is. If it were me I wouldn’t worry about the red if I just ate something with tomato sauce, but if you are in pain I would say you should address it with a doctor. If you have a fever that warrants an emergency.
That’s my non-medical, no way for me to really know your situation, opinion.