I Have Been Experiencing A Racing Heart (140 To 170

I have been experiencing a racing heart (140 to 170 beats per minute) after minimal exercise. Just walking up and down a flight of stairs one time. I sit down and my heart rate decreases to between 70 and 90 beats per minute after about 5 minutes. I also have some shortness of breath when I heart rate is so high. I am also extremely tired all the time and have little energy. Normally I can walk 2-3 miles and have no ill affects. This has been going on for at least a month