That's interesting. So, you know that your bp drops because you suffer the symptom of light headedness or syncope? Now, cardiologists make their living worrying about our heart and health so would try to trust them. One of the main reasons, even when on bp meds is something called hypovolemia. It's reduced blood volume and one of the main culprits of what you describe. Guess what the main thing that causes this is? Dehydration. Good old dehydration. Also if you fast, have diarrhea, are in the sun. All of these things can cause it. It normally rectifies itself with no issue and is not serious. Track what happened before if you have an episode again . . .what you ate, where you were, how much you drank. Try to see if you have a pattern. This article has a lot of possibilities but since you see a cardiologist, I would guess they have determined it is of the benign type as above. https://www.verywellhealth.com/sudden-drop-in-blood-pressure-1324162