Good question. Of course, no one can fully answer this question with giving you a full check up, so I would calmly get yourself checked out. However, it sounds like what you are experiencing are heart palpitations--a generally harmless phenomenon that are often a product of stress. The fact that you said it "almost feels like an anxiety attack" makes me think you are right. Many people go to the emergency room--it is 2/3rds if i remember correctly--thinking they are having a heart attack, when it is just stress or anxiety related issues. The fact that you are not "nervous" when you feel the "thumping" does not mean it is not stress related. Your body could still be experiencing an abnormally high level of sympathetic nervous system activity, and adrenaline, even if you are not at that moment doing something that you find stressful. I would recommend looking at the level of stress in you life in general, and try decreasing the stress, or improve your stress management. And of course, still get check up by a medical doctor as a precaution.
Anthony Centore Ph.D. -- Online Counseling, Telephone Counseling, In-person Counseling. www.ThriveBoston.Com