I assumed i had a yeast infection and started taking monistat 7-day treatment. after the first use i noticed small pink tissue-like discharge, different from blood. could this be tissue from inside my vagina? what should i do and what could that be?

Vaginal discharge: You don't say what symptoms made you think you had yeast. The main vaginal yeast symptom is itching; if any discharge, usually small amount and thick, with no odor. Colored discharge (yellow, green, "pinksh", etc) usually means an infection of the cervix, and at your age STDs (gonorrhea, chlamydia, trichomonas) are the main causes. Stop the monstat and see a doctor ASAP. Good luck.