What Causes Episodes Of Fainting And Convulsions?

Hello! Two years ago I used to faint in the morning maybe a couple of times a month. Then there was a period of about 6 months when I was fine and it didn't happen, but then around this time last year it started again but less often. And now it's no longer only early in the morning but in the afternoon or in the evening too and I started having convulsions as well. I've been to the hospital and done blood tests and brain scans but the doctors could only tell me that it's because my blood pressure drops, but they don't know why it drops. I really want to find out why this is happening to me! Thanks