Skipping heart?

Hello i am a concerned 25 year old male. I smoke i also use heroin on a casual basis. (i  know thats not exactly healthy but im being honest) In the past week i have been noticing my heart seems to skip a beat and then double beat and give me an urge to cough. does this all the time. after four days i went to the hospitol and told them everything. then ran many tests everything came back normal and it didnt happen while i was there. went to the doctor today for a follow up. she listened to my heart again it didnt happen. she assured me i was fine. however iit is very distressing and still doing it, i dont have great insurance i am young And otherwise healthy but when i was in the hosp my heart rate was in the 50s and im just concerned. ive been reading all this stuff on arrythmias and pvcs and im just scared and i dont knwo what to think. EKG turned out ok. as did my heart monitor and bloodwork and xray. should i pursue an ultrasound or holtermoniter?...or should i believe what they say that it is nothing...i would like to know foor certain that it is nothin to worry about. very distressed.