Aborted swim :(

I'm hoping I haven't done anything serious and/or daft - yesterday I was crewing on the wherry (yes, the very boat I am supporting by doing this tri!) and in amongst all the shoving, heaving and other quant-related activity I seem to have pulled, twisted or otherwise slightly knackered something somewhere. There's a niggly little pain under my armpit when I breathe in and do some movements, although it's not really bad. However, when I swam today, only ever intending to do a relatively gentle swim and concentrate on lengthening my stroke, I could only manage 15 minutes - breathing in with right arm extended (as I do at the start of every length) was not comfortable at all, and so in the end I gave it up as a bad job. Still, I only did it yesterday, so maybe if I rest it until the lake on Saturday (or beyond if it's not much improved) then it'll be OK. I hope! If push comes to shove on race day I reckon I can grit my teeth and get through it, but it'll be annoying if I can't do as well as I want to because of this.