High heart rate or tachycardia can be due to low BP, anemia, hyperthyroidism, fever, low blood sugar, anxiety and panic disorder, heart causes
like sinus tachycardia
, alcohol, coffee, effect of certain drugs etc.
If you are aware of this high heart rate then this is called palpitations. Palpitations may be normally felt if you are overexerted. Chemicals like adrenaline, alcohol, caffeine, cocaine, amphetamines, and other drugs all cause palpitations.