I absolutely get this, especially before MDMA. I can need a shit just thinking about a night out. I've been sick loads of times, before dropping. I've gagged and instantly thrown up loads of pills. I put it down to my anxiety going into overdrive, in anticipation. It seems it also happens to people without anxiety, but MDMA seems to be the most common drug for this to happen with. Loads of mates are the same, mainly ones who have hammered it. I think your brain starts releasing serotonin, etc. before dropping, so you get the side effects early. Once you're up, it seems to fuck off. It is definitely anticipation of what is coming. The same way you can make your body tingle, like MDMA would, by listening to the right music. I'm sure others get this too. Placebo at its best.
I find things go better when you've not spent a week thinking about it, and it just happens. In a way, it feels like my brain hates MDMA, on an unconscious level. I'm not surprised.