What can cause tachycardia and palpitations, cold all the time and weight loss? it's not my thyroid or diabetes.

Many causes: Damage to heart tissues from heart disease, abnormal electrical pathways in heart, congenital, high blood pressure, smoking, fever, drinking too much alcohol or too many caffeinated beverages, abuse of recreational drugs, such as cocaine, imbalance of electrolytes, overactive thyroid (hyperthyroidism) in some cases, the exact cause of tachycardia can't be determined. See physician for workup.

Check up: Need evaluation for arrhythmia/ a holter monitor to start with and check for low BP evaluate for chronic fatigue syndrome.

Syndrome: Excess catecholamines is one possibility, have you discussed this problem with your doctor?