Final answer:
The symptoms are most likely due to a decrease in blood pressure caused by taking nitroglycerin.
Step-by-step explanation:
The symptoms described, lightheadedness and feeling as if one is going to faint, are most likely due to a decrease in blood pressure. This can occur as a side effect of taking nitroglycerin. Nitroglycerin is a medication commonly used to treat conditions like angina, which is caused by reduced blood flow to the heart. Nitroglycerin works by relaxing and widening blood vessels, which can lead to a decrease in blood pressure. It is important for the woman to sit or lie down and seek medical attention if her symptoms persist or worsen.