Afterwards, Y. told me that I had nothing to be worried about; he followed the whole thing, it was fine. But in the reciprocal situation, he is often nervous and I always have no trouble following.
I've experienced the same effect in writing; it's much harder to write in my field that outside. (I wrote some terrific English, history, and opera papers!) What it amounts to is this:
The cost of failure is very high.
If I botched a paper in a humanities course, it meant very little to me; it was not a field where I had declared any interest or professional intent. Compare that with now, writing my master's research paper: every word seems heavy with meaning, and not just because the topic matter is dense. It has import; the stakes are much higher, since this is something I want to do well in, something that affects my planned career path.
Unfortunately, the only way to overcome this performance anxiety is to just keep writing and giving presentations until it becomes natural.
This post's theme quote is from Johann Wolfgang von Goethe:
Mathematicians are like Frenchmen: Whatever you say to them they translate into their own language, and forthwith it is something entirely different.