IFJane, i too got the stitch in my side. But it took me to be 50 years old before i understood why. I was out of shape! i was not conditioned. I spent too much time sitting around and drawing horses. and reading. I was a rather sedentary child. So... exercise HURT, it was no fun.
I am now more active than i ever was! (how strange) and it took a few years of riding before i got the energy to sustain.
I used to average 9mph and just plain run out of steam after 5 or 10 miles. My puny legs couldn't take it.
So maybe that's the answer. I am now strong enough to work harder, so I was breathing too hard? and roughed up my throat?