I've been struggling with something for a little while - and avoiding talking about it with my friends because they just tell me to get more sleep

I'm 25 and 2 years ago I started experiencing sudden periods of extreme fatigue. At first it was only happening once or twice a week and only when I was sitting down at the computer, then it began being every day, and more recently it is multiple times a day and can be when I am sitting down, talking to someone, or even walking around.
I would describe the feeling as extreme debilitating exhaustion, I have to fight really hard to stay awake, I get double vision from trying to keep awake, I get shock-like feelings in my arms as if they have fallen asleep and are jolting awake, my speech goes slurry, and my brain feels like it stops functioning properly. It is getting to be quite a problem at work because I don't really remember anything that happens during these periods (so it can be people explaining a task to me, and I will remember none of it immediately afterwards). I have also started to almost fall off chairs (which is quite embarrassing) as my body jolts awake - which I presume is from momentarily falling asleep?

The amount that it happens doesn't seem to relate at all to the amount of sleep I get the night before - I can be completely wide awake and then all of a sudden I will crash and be struggling to stay awake.
Also the fact that it is happening more and more regularly, and increasingly when I am standing or walking around, is beginning to worry me - because I work on site quite often where I really need to remain professional!
Does this sound like it could be narcolepsy? Could anyone suggest anything I could do to try to reduce these attacks of exhaustion?
Any help/advice would be really amazing!