pelvis/abdomen pain?

Em
I have been having my period for over 3 years now (I'm 15) but occasionally I get a super sharp pain from my stomach down past my pelvis. The pain varies and is super random, but when it happens most of the time it feels like one sharp knife stabbing through. I am not even able to walk! 
I have no way of relieving the pain, and usually have to trying and wait it out. The pain shoots sharply for maybe 10 minutes at first, fades away, but then stays for another half hour or so, coming back harshly when I try to walk or even press down on my stomach lightly. 
Does anyone know what is going on? 
(Before you ask, I am 100% a virgin, pregnancy is 100000% out of the question) 
Help!! 

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