Anyone please help me with breathing question?

I am a 27 year old male...I used to weigh about 300 lbsabout 5 years ago...Through extensive running I am now at 175lbs..I run approx. 40 miles per week...Since the weight came off I have noticed I get some irregular heart beats every now and again (fluttering) And I can ALWAYS feel my heart beating...Recently (the past couple of weeks) I have been experiencing a difficult time catching a deep breath...And this scares me...I find myself having a very tough time when I'm running completely catching a deep breath...Now I do not get winded mind you...just can't catch the deep one...Now I know I am stupid because I smoke cigarettes and have tried to quit...and I definately drink much more than I should...I had a cardiac cath done two years ago and everything was fine...I guess my question is should I be concerned about this breathing pattern over the last two weeks...Could it be heart related? Or could it be something that is a concern in my lungs...I am apprehensive to go to the DR. because I have been there so many times in the past two years with heart related concerns and they never find anything...I also take lexipro for mild anxiety...Any help would be MUCH MUCH appreciated