I definitely agree with the previous comment. I am 18 and experience similar things sometimes after my period. Hopefully you can relax knowing that its not an uncommon thing, I know many younger women who experience this. Although, you may want to visit a doctor if it truly worries you and especially if you have abnormal pains.
It's most likely old blood that wasn't discharged completely with your last period. If it still persists, you will prob have to go to the dr. to make sure it isn't an infection.