Hi,clots seen in menstrual blood may be normal. it may be due to-
1.tampoons you use. because the blood is blocked for a while it gets clotted.
2.fibroids in uterus
. fibroids or polyps in uterus are not uncommon cause of clotting of menstrual blood especially at your age.
3.endometriosis-though uncommon at your age, it might be a possibility.
Try to see a gynaecologist nearby you who can rule out these using sonographic guidance.hope that helps. take care! :)