I'm not sure what that could be. I've been training for a marathon and i've felt all kinds of stuff recently. I don't remember that particular pain being something i've felt. Even though you think it may not be a stich, I wonder if you should try to eat a bananna to increase your potassium level and see if that helps. It could be a stich, but just a stich that you haven't felt before.
As far as breathing goes, my dad used to tell me to take longer breaths if I started feeling a pain in my side, but I think that was more for preventing cramps.
I would go see a doctor about it, to be honest.