hi , i am 30 , if i stand up a bit too fast or something similar i can fell my heart pumping in my chest and change rythmn  and people get little pains in their body and because of the heart changing rate and i can feel the pressure i get worried also about little pains in left arm when i get them but this might be my sleeping position but still i get worried about the heart thing if its A F   atrial fib or something similar , ive been taking a low dose aspirin just in case and am making doctors appointment tommorow  , i do not drink but smoke sometimes and am overweight but not obese im 6 foot and around 15 -16 stone i think , i am going to radically change lifestyle and diet and exercise , but am hoping you could share your insights and perhaps help calm me down a little as when i think about it i get stressed and panic a little , thanks , p.s im on a low dose of medication for previous opiate misuse  ,but havent used any class A substances for years
Most of the time with all what you describe might just be plain increase in heart rate which is physiological, and you may perceive it more because of associated anxiety.  Just relax during such episode, curtail your mind, and it goes away mostly it is anxiety related.  Now if the clinical suspicion is strong then get a 24 hour holder if the symptoms are every day.  An important note here is if you take more than prescribed dosage of your meds, there may be transient increase in heart rate and hence please don't do the same.