Heart beating too fast?

I was diagnosed over five years ago with mild pulmonary hypertension. I have been doing extremely well, spin cycling, long distance walking, etc. I had a recent sonogram , EKG work up etc. I had a excellent feed back other than what is wrong. I also did a heart testing while walking on the treadmill. My target rate was 46, I went up to about 72. Just today, for the first time ever..., I noticed my heart pounding so hard that i saw it vibration outside of my shirt. At first it started beating fast then the actual visualization, and eventually it calmed down. Prior to the horrible event, i had eaten a grill cheese for breakfast. I then walked up the train stairs, about 50 steps. While walking i usually take deep breaths, in through the nose out through the mouth as i find this easy while I am climbing the NYC train stairs on a daily basis. I always try to remain relaxed and breath free when walking. When i first sat down, this is when I experienced the problem. Could this be due to the deep breathing or the cold weather? Or could it be related to the condition-the doctor told me i shouldn't have any problems?? or could it just be anxiety attacks out of no where? Help Thanks