Sharp stomach pain please answer?

[Marco]

I'm a 17 year old male been having this problem for 3 years it doesn't hurt everyday but it happens at least one time every month. When it starts hurting I have to be sitting down still because if I try standing up straight or try to pee I get a sharp stabing pain behind my belly button and it hurts like hell that I can berly walk. The pain usually goes away after 3-4 hours. I have been to the doctor already but they didn't find anything wrong with my blood and urine. Does anyone know what's wrong with me? If you need any additional details let me know and I will reply. Thanks