Thank you for your question. As others as stated you certainly should be able to lay on your stomach after you have fully healed from your breast augmentation. Soft freely movable breast implants should not cause you discomfort. However if you experience discomfort when lying on your stomach he should avoid it.
Consult with a Board Certified Plastic Surgeon for an evaluation, and to discuss your concerns and expectations. My surgical assistant ran over gel implants with an SUV, and they did not burst. Breast implants undergo vigorous testing for safety and strength. Best wishes!