I think I might have a cyst????!

I am 15 and had my first period ever on jan. 30th it lasted 9 days until february 7th. About 3days after it ended I started cramping uip again and getting shar stabbing pains in my lower abdomen for 4-5 days I've had alternating light brown and blackish brown discharge it hurts too pee and poop I still have stabbing pains I'm neauseas all the time and I have a lot of aching pressure down in the vag area and i don't think this is normal... Could I have a cyst?? Or. What's going on? Should I see a doctor??oh and I'm not sexually active so its not an std