Well there are various reasons why this could happen...
#1 - You drank too much coffee/or other types of caffeine
#2 - You took some type of medication that could speed up your heart, ie: cold
#3 - You are having a panic attack
#4 - You've eaten too much...
#5 - You have drinken alcohol the previous night...Causing heart palpitations the following day...
#4 - Some other type of health-related issue that you may need to get checked out, if it keeps persisting...See your doctor