Hello, At your age, it is common to have some amount of cycle disturbance leading to heavy or sparse bleeding as the body tries to adjust to the hormonal milieu introduced after menarche. The size of the clot determines how much collected blood has stayed in the vagina before being let out. You should see a gynecologist for a proper evaluation and further suggestions. Investigations like blood counts, bleeding and clotting profiles, sonogram of the pelvis and urinalysis usually suffice. Correction of anemia and other nutritional disturbances, styptics to arrest bleeding, rest during periods, avoidance of stress and finally a few courses of combination pills help to overcome the problem. Hope this helps.