Of note, vitamin B12 is naturally found in animal products, including fish, meat, poultry, eggs, milk, and milk products; it is generally not present in plant foods, but some breakfast cereals are fortified with vitamin B12 in a form that has a high bioavailability for vegetarians.
Another equally likely and probably co-existing possible cause is that of iron-deficiency anemia: at your age, you are still in the very active growth phase and menstruating (blood loss through menstruation without adequate intake of rich sources of iron from the diet results in anemia). Iron is an essential component of hemoglobin, the main red blood cell protein that transfers oxygen from the lungs to the tissues. As a component of myoglobin (muscle protein), a protein that provides oxygen to muscles, iron supports metabolism. Iron is also necessary for growth, development, normal cellular functioning, and production of some hormones and connective tissue. Dietary iron has two main forms: heme and nonheme. Plants and iron-fortified foods contain nonheme iron only, whereas meat, seafood, and poultry contain both heme and nonheme iron. The RDAs (recommended daily allowances) for vegetarians are 1.8 times higher than for people who eat meat. This is because heme iron from meat is more bioavailable than nonheme iron from plant-based foods, and meat, poultry, and seafood increase the absorption of nonheme iron. So again, if you are not meeting those requirements, you are highly likely to have iron deficiency. I strongly recommend that you do a complete blood cell count (CBC) and iron studies (ferritin, iron level, total iron binding capacity TIBC) to rule out this possibility and initiate iron supplementation if necessary.