Unless you have a seizure disorder, this probably isn't a seizure. However, if you are feeling what you imagine a seizure feels like, it is very possible that this IS an orgasm for you. That is often how they can feel. If you are experiencing that, but feel like that can't be an orgasm because you don't feel satisfied, or "finished," then it's possible what's happening is just that your body is in the game here, but your head is still very focused on trying to make this happen, on your frustration with the whole thing and other distractions that are keeping you from really just having the experience.I'm positive it's not an orgasm and my body just kinda has a small seizure if I keep touching myself after it happens.
It's also worth saying that orgasm that feels fuller and richer, rather than say, like a little bit of an electric shock or a tiny seizure, tends to come from whole sexual experiences that are fuller and richer. In other words, to get to experiencing orgasm as something that leaves you feeling more satisfying, the whole experience often needs to be longer, more involved and gradual, and again, not focused on orgasm. Orgasm is just a few seconds of sexual experience: what gets people there, and leaves people feeling satisfied, is not usually the orgasm itself so much as the much longer whole experience. Does that make sense?
Lastly, you also may be right that what you are feeling is NOT orgasm. If not, what it could be is just how it can feel when you are overstimulating a certain area. If and when that happens, it can help to change what you are doing or where, so, that means doing things like pressing more softly if you were pressing something more firmly, or switching from touching right on the clitoral head to taking some time with more general touch of the vulva, like with the outer labia, mons or clitoral shaft.