You locked your knees without realizing it, shut down the blood return from your legs, then had a drop in blood pressure due to decreased blood return to your heart causing you to pass out.
You took a big hit, expanded your chest, held it, decreased blood flow through your thoracic cavity due to the increased intrathoracic pressure then a blood pressure drop as above.
Your vasovagaled yourself somehow (bearing down on a held hit maybe or just due to weed effects) which is increased parasympathetic nervous system tone that drops blood pressure
Some weird shit 🤷🏼♂️, the body can be odd and changes as we age so maybe you just can’t smoke weed now cause of the earlier mentioned weird shit
I agree with your doc (I’m a family med physician), don’t smoke if all of a sudden you’re passing out.