I really don't think that it is anything to worry about, but it could be that when pushing to pass urine whilst standing that you are somehow pulling a muscle around your belly button that lets you know its there from time to time.
Do you ever have this problem whilst sitting on the toilet to open your bowels? It is just that when you have a BM and also urinate, you need to push a little so that the pressure in your belly rises so that you can open the necessary sphincter muscles, being the bladder for urine and the anal sphincter for stools to pass out. When you push, the pressure in the belly rises allowing this to happen.
It could be that when standing to urinate you notice this more than when sitting for a BM. Whilst standing your belly muscles pull in a slightly different way.
Don't worry, but hope that this may help?