If I'm feeling very anxious or overwhelmed, it can effect my speaking in all sorts of odd ways, and one of them is very like you describe. I'll breathe in deeply when I realise that I'll have to speak, and I'll quite often hold it for quite a while if the other person is still finishing. Then I try to get the whole sentence out in a single breath, getting squeakier and squeakier as I run out of air - followed by a gasp for the next breath, usually. I have to know exactly what I'm going to say beforehand, and to have practised in my head, too - once it get's that hard for me to speak, I can't decide what to say as I go along at all.
It's not often for me that it's that bad - but there's kind of a scale between being able to talk fluently and that, with a bit in the middle where I pause, stutter, repeat bits, forget words, and that kind of thing.
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