left arm pain along with diffuse chest pain and palpitations is likely to be cardiac in nature... please note a one off ecg may not pick up the cause of palpitations....the suspicion of cardiac disease is heightened when you said you had shortness of breath during sleep...so here is what you should do...try to get a HOLTER( an ambulatory 24 hour ecg done) and also an echo cardiogram .
as for the red blood in the stool if it persists and defecation is painful it is likely to be a local cause like a fissure in ano... also please note if the colour is red or really a dark red..because that may be useful in determining the site of bleeding... however in the meanwhile, you should take a high fiber diet and plenty of fluids to avoid constipation, it should help....
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dont get distressed !...watch whether the pain increases in intensity, if it is constant you can wait ...but if it is increasing in intensity call 911...there is no harm being persistent with the ER doc, after all it is your health!... and do try some local application like diclofenac gel over the left pectoral and see if the pain decreases, then most likely it is musculoskelatal and not cardiac in nature....as for the stool being bright red the most likely site of bleeding is the lower gastrointestinal tract... watch for a couple of days...if there is still blood in the stools, meet a doctor, in all likelihood it could be a fissure in ano if painful,..... all the best vivek