“What problems can a septal defect cause if left untreated?”

Depends on the size of the defect and the site of the defect. It can cause irregular heart beat and heart enlargement and heart failure in the long run.

Stroke

There could be several different types of “septal defects” in the heart. It depends where the defect is, how big it is, is it associated with other potential abnormalities in the heart, is it congenital (at birth) or is it acquired (occurs later in life). Many of these defects can close spontaneously throughout the life. However, the larger ones can persist. If a “septal defect” persists and it is large enough to cause significant left to right shunting which overwhelms the right side of the heart and lung circulation, it can cause high lung pressures and right sided heart failure. In addition, once lung pressures increase high enough it can start shunting blood in opposite direction, meaning from right to left, decreasing the flow and oxygenation through the lungs, now leading to a decrease of oxygen in a person’s circulation and tissues. This can cause bluish discoloration of skin, shortness of breath, fatigue and other symptoms. There are other potential risks of having a septal defect, including blood clot migration which could lead to stroke or infections. It can be easily diagnosed by heart ultrasound or CT scan or cardiac MRI. The degree of shunt and direction of the shunt can be measured. Sometimes, heart catheterization may need to be done to measure pressures inside heart chambers and lungs, and to determine the size of the defect. Based on patient’s symptoms, the size of the defect, lung pressures, it can be determine whether the defect should be closed or not. You should be referred to a cardiologist for a comprehensive evaluation.  
Good luck!