How does coffee affect you? Seeking advice from diagnosed individuals.

How does coffee make you feel? I’m undiagnosed and am going to talk to a doctor at the end of the summer. One (among many other things) that tipped me off is that I can drink coffee at any time of night and not be jittery, in fact sometimes coffee makes me sleepy. But after working in a coffee shop and drinking something like 4 nitro brews a day, something switched and now, not a lot but ever so often, coffee will make me alert to the point of a headache. I don’t really understand how I can most of the time be unaffected and then sometimes be on either extreme? If you’re diagnosed please let me know!