Can you loose calcium in you teeth which causes yellowing? i haven't been eating a lot and my teeth got extremely yellow.

possible: One scenario where this would be true is when an issue such as an eating disorder or rampant gerd might allow stomach acid to dissolve surface enamel from teeth removing a white covering over underlying yellow dentin. More commonly, poor oral hygiene and diet choices cause extrinsic staining which can be controlled by getting regular professional cleanings. A dental exam will help you understand.

Not usual culprit: Tooth discoloration at your age is primarily due to a combination of the foods that you eat, a compromised Oral Hygiene program, and too infrequent professional cleanings. Discuss your tooth color concerns with your Dentist/Hygienist.