This kind of episode can come out of the blue---any of the things you mentioned could contribute to this happening. How were you afterwards? Did it pass as quickly as it came on? It is good that you do not associate this with public speaking, do not avoid doing it again.
Also. This might seem like an odd question but did the others watching you see what you felt? Did anyone say anything to you?
When young, I developed inexplicable, sudden phobias of things I had never feared. I did find that forcing myself to ignore the fears helped dispel them. (ocean, I jumped in, horses, I rode again, dogs, took a while but once I got one the fear left for good)