Why do I have this twitchy, spasmy reaction to some pot?

Sometimes, when I smoke pot, I get twitchy. And spasmy. But like, really teeny, tiny spasms. It’s like smaller parts of muscles – smaller than I can control on my own – are just twisting into knots. I had a similar, although much more serious, reaction to being on Topomax.

But here’s the weird thing: It doesn’t happen every time. If it happens with one batch, it’ll always happen with that. But I don’t know if it’s the strain, or perhaps the fertilizer used, or what. I’ve had both Indica and Sativa ones that don’t do it to me.

So, why is this happening, and is there a name for this reaction, possibly with a corresponding Wikipedia page?