Hello, I have gone through your query and understand your concern. As you suspect, either missing the dose or propranolol itself, can cause Shortness of breath. But for you it is associated with chest pain, it cannot be taken as simple. Because this chest pain and breathlessness may be due to Heart attack. In fact , you did not missed the dose and took some time later. Thus you need further cardiac evaluation immediately to rule out any coronary heart disease. In ER, they may check your blood pressure and pulse. They will take Electrocardiogram to rule out any Angina or Heart attack. Iit is better to rule out by investigation than waiting for further devastating condition and experience. Hope this helps. Please write back for further queries. Wishing you good health.