If you have had a thorough evaluation by your doctor, and no medical causes for your symptoms were found, you may be suffering from panic attacks. Panic attacks are characterized by abrupt, recurrrent, and short-lived symptoms such as palpitations, chest pain, sweating, trembling, shortness of breath, e.t.c. Palpitations in anxiety or panic attacks may be mediated through increased neurochemical discharge from certain brain cells, causing an increase in heart rate.
Panic attacks are treatable with a combination of medications and cognitive-behavioral therapy. You may find it helpful to seek evaluation by a psychiatrist.