I Have Had Two Experiences Where My Blood Pressure Bottomed

I have had two experiences where my blood pressure bottomed out. The 1st time, I was standing in line at a concert waiting for the gates to open. I was tired and hurting. I have degenerative disc disease. Standing for even a short period of time causes pain, I had been in line about 45 minutes. I passed out. ER said I was dehydrated.
2nd time I was in my Doctors office. It was winter but I suddenly felt hot. I was fanning myself. When I started to leave I walked to the next room and I was talking to a lady. I said "is it hot in here to you?" She said "no". Next thing I knew I was lying on the floor with my feet elevated. The doctor said my bp had bottomed out and he didn't know the cause for it.