Real big stones are really painful and usually have to be either crushed by laser or removed surgically but if they are small then they don't cause too much grief unless they get caught in the ueter(sp?). They come in different shapes and sizes too. To know if you have a kidney stone, you first need to see a urologist and they can do a CAT scan to see if you have one.
Edited to add: if in fact you do have stone, which btw as a Uc'er you are more prone to them - this is another extra manifestation of Uc, that a small one will be passed thru the kidney, into the bladder and then thru the urethra. I heard that it can be extremely painful when passing a stone. If the stone gets caught in the ueter (which is the opening from the kidney into the bladder), then it can cause bleeding and pain and requires immediate medical assistance.