Narcolepsy with cataplexy is an autoimmune disorder. As I understand it, no one yet knows the cause of narcolepsy without cataplexy. Unfortunately, in most of the articles written about these studies, the authors will only make reference once in the beginning to the fact that they are studying, specifically, narcolepsy with cataplexy, and that confuses the issue. I've more than once had to re-read some of these studies to know whether they're studing narcolepsy with cataplexy or narcolepsy without cataplexy--and it almost always turns out to be narcolepsy with cataplexy. I've found very little research on narcolepsy without cataplexy, beyond the studies that established there may be at least one different cause for narcolepsy without cataplexy, unrelated to the autoimmune reaction they now believe causes narcolepsy with cataplexy.
I just had a recheck with my doctor, and asked him about cataplexy, since it's a subject I don't have much understanding of and I was wondering about one particular incident in my own case. That incident involved me standing in a line at a store, mildly annoyed because there was only one register open and the lady in front of me had three carts crammed full of stuff. While standing there, I shifted my weight--and just fell over. I sat there for a second, surprised, then stood back up again. I blew it off--I have weak ankles, and it isn't the first time one gave out under me--but others here suggested it might have been cataplexy. My doctor thinks not, however, because there was no "strong emotional trigger." And, frankly, if I collapsed every time I got even mildly annoyed at someone, I wouldn't be able to walk. So, from what the doctor said, unless there's some sort of strong emotion that might be triggering the attacks, it's unlikely to be cataplexy.
Note, I did not say it isn't cataplexy. There are people here with a lot more experience with cataplexy that will hopefully be along to give you some advice. But my understanding is that, while you may feel more sleepy after a cataplexy attack, and being sleepy might increase the likelihood of them occuring, sleepiness itself is not a trigger. So, my advice would be to think about whether or not you've experience any particular emotion--not sleepiness, but emotion: happiness, sadness, laughter, anger, etc.--immediately prior to your neck feeling weak. I know, in my case, when I have a sleep attack, it's awfully hard to keep my head up and my eyes open ... but it isn't cataplexy, it's just my body wanting to lie down and sleep.