OMG! I'm 27, and I have also had this my whole life. I haven't had it in several months, and today I got out of bed really quick and I almost died. It's like a hook pulling on my belly button. Sometimes I notice than I get it more often when I'm about to get my period. I've had ultrasounds done over the years and no one can seem to explain this. I just sort of wait for a couple days to pass before it starts feeling better, and you're right, once you get the pain it seems to get bad when peeing, bending, stretching and making sudden movements. I have been checking forums for about 10 years to see if anyone else's doctor has been able to find the answer to this. Seems like we will have have to keep sitting tight. :(
Alright guys, I get this sometimes too and that's what I think about it. Remember I'm not a doctor so I might be wrong. I did talk to a doctor on some medical forum though and he said I might be right.
So basically there's aponeurosis (google will explain what it is better) between your belly button and genitals. And it can be damaged, that's how hernias appear. Usually it's damaged by lifting weights but I think it's also possible to damage it when straightening or turning your body too fast. This explains why some people in this thread got their hernias treated and the problem was solved - the hole in the aponeurosis was sewed up. I don't know if it's something you're born with or you damaged it the first time it happened.
What to do? First, go get an ultrasound of something like the anterior abdominal wall (if you get an ultrasound of the internal organs, it just won't show what you need). If it still doesn't show anything, either my theory is wrong, or the hole is too small, or you have another cause (I think what we're experiencing is a symptom, not a disease, which means the cause might be different for each of us).
Another possible cause is the lack of physical activity. Sitting motionlessly isn't natural for the human body, so it deforms your muscles and everything else so when you stand up too fast, they can't get in place like other people's. What to do? Exercise. Don't lift weights, don't do anything too hard, just simple gymnastics, maybe swimming, STRETCHING (mostly abdominal muscles). Be very careful, don't push yourself too hard. And obviously don't try to exercise during these episodes (I know that's impossible because of how painful it is, just don't think I'm suggesting it). For this theory also speaks the fact that in my case this thing stops after I go to sleep, I think muscles relax during sleeping and get back into their place.
If nothing helps, try passive treatment: just don't stand up fast, don't lift weights, basically don't do anything that puts any kind of pressure on your abdominal. That won't really limit your life too much but will spare you a lot of pain.
P.S. English isn't my first language so sorry if something sounds weird. Hope this helps at least someone.