I hope you give a rest to this now. You don't get it still - no matter what people say their 'first symptoms' were, those diagnosed with ALS tend to have clinical findings (which indicate failures) from their first visit. You are not presenting with clinical findings, but are describing all the things you feel and fear. That is the difference. I was also referring to 'earliest symptoms'.
Yet, most people with a headache don't jump to brain cancer, even though that is likely to be one of the 'earliest symptoms'.
So what you are describing could be dozens and dozens of other more common, and more likely things. Please just go work with your doctor now.