A yeast infection is usually itchy, the discharge looks like cottage cheese or cheese curds and has a bad odor. Vaginal infections are tested for by taking a vaginal swab. Were you having the discharge when you saw the doctor? If so, did you tell him/her? If so, what did the doctor say?
Yes. It would show up on the test. If it is STD or anything, the blood test will also indicate some clues. If blood test is not enough, saliva test, ultrasound, and X-rays will be also helpful. For a full list, go to the clinic and see the list of the possible tests that medics can do there.
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Basically you have secondary amenorrhea or absence of periods/menstruation due to some thing. The first thing to check for is pregnancy (even though you are so young!), so take a pregnancy test and see. If this is not the case, then other causes will have to be looked into. Stress and anxiety is a major factor. Thyroid problems and abnormal levels of FSH, LH, prolactin, androgen hormones or male hormones and estrogen hormones too can be the cause. MRI of brain may be required to rule out pituitary tumors secreting prolactin. Another major contributor is polycystic ovarian syndrome (PCOS). This can be diagnosed by a CT scan or abdominal ultrasound and tests for insulin resistance. Smoking is also often associated with secondary amenorrhea and can be worse in presence of stress. Sometimes tumors of the ovaries and brain may also cause absence of menstruation. Severe anemia and malnutrition, excess weight gain or weight loss, hypothyroidism too can contribute. If you have been on pills or depot shots then this can be post pill amenorrhea.
Your mother needs to discuss all this with your gynecologist. Take care!