A course of antibiotics disturbs your gut bacteria for weeks afterward, and that, on top of your IBS, commonly causes exactly this trapped, bloated gas feeling. The antacid you're taking helps with acid but doesn't really address gas.
Next Steps
Try smaller, more frequent meals for now, and cut back on carbonated drinks, beans and very oily food. A short walk after meals and a probiotic to help rebuild your gut bacteria can both speed this along.
Health Tips
See a doctor if the discomfort turns into sharp, worsening pain, or comes with vomiting, fever or blood in your stools, since that needs a different work-up.