As someone who had a petit mal & at least one grand mal in my early twenties I will add my pennyworth. The cause was never identified but the EEG showed abnormal brain waves when I was sat in right front of a strobe TV screen.
Firstly loss of short term memory of the immediate events before the grand mal fit is quite normal. Your fit sounded not unlike my first grand mal - a very stressful day and possibly not enough fluids.
I was treated with a barbiturate based drug (this is 45 years ago) but when I moved to the middle east for work I had to come off this medication due to the local laws. When I came off I found that for me there was a direct link between my epilepsy warning symptoms & dehydration. Alcohol in particular caused me problems which I discovered by trial and error to be linked to loss of body salts - using the oral rehydration powders sorts this out immediately for me. In the absence of rehydration salts, lucozade or something similar also helps.