i think its your period coming, sometimes i have kind of the same thing. my friend is 13 or 14 and shes going to specialists, she gets her period shes only had it like 3 times its so spaced out and they are trying to figure out why...so it could end up being serious, go to the doctors.
This might be the beginning of your period or could be from another complication. Have you been to your doctor about this? I would make a doctor's appointment to find out why you're not having a period. You want to find out what's going on to make sure you don't have any hormonal complications or another problem. Once they find the cause, they can kick start your period for you and get you back on track. Hope it all goes well.