Are you able to check your blood pressure at home? One of the potential side effects of the med you're taking is sudden drop in blood pressure, and that correlates with the symptoms you describe.
I do have a BP Omron device and occasionally use it. The only abnormal reading is 30-40 minutes after sex the heart rate goes up substantially. The systolic/diastolic is normal at a 120/80 range. But the heart rate goes to 115-120. This corresponds to the period of weakness, etc.