Hi,
Palpitations are most likely to be due to panic attack, but they can also signify a heart disease like ventricular tachcardia, etc.
To distinguish them, you should do a simple test.
Take rest for 15 -30 minutes. Then, check your radial pulse, the pulse in your wrist. Keep a watch nearby and count the number of beats in 1 minute. Now, note these things:
Rate: The number of beats per minute. It should be between 60 and 90. As long as it is in between, dont worry.
Rythm : The beats should be regularly spaced. The beats are a bit more fast when we breathe in and a bit more slow when we breathe out, thats normal. But beyond this, it should be generally rhtymic with the space between two beats being equal.
Force: The pulse should be firm and strong. Each pulse should be equally strong. You should not have some beats which are thready and others more strong. (this may be difficult to distinguish though for those not trained to check pulse).
As long as these factors are normal, dont worry. Your heart is beating normally, its just that you have become more conscious of your heart beat, thats all.
If you find any abnormality, then you have to approach the doctor.
I hope this answer was helpful. If you need more information, feel free to ask. For accepting this answer, please click on the green 'accept' button below. Feedback/Bonus will be warmly appreciated.