Hi Erin, I'm Dana and had a form of tachycardia called VT which is in the lower heart muscles or the ventricles. So the word tachycardia only means elevated heart rate and depending how hi and where it is will determine what the problem is. Typically around 70 bpm in that rage at rest is normal and during exercise can easily get up to 150s so I won't get concerned yet. Now the out of breath is a concern as when you heart speeds up incorrectly, oxygen rich blood is not getting to the needed areas hence why it seems that way. Your heart speeds up to get more oxygen that's needed. So it great your getting it checked out.
Now for things I wish I knew when first diagnosed was options and outcome. But my first thing i would suggest is depending on what the tests show get to the correct specialist. The Heart is so complex that doctors today specialize. The basic areas are what I call plumbing like blocked arteries and veins and those are your general cardiologist
But for heart rhythm issues you see a EP cardiologist the ep is electro physiologist or the electrical parts of your heart and even there they split between upper(afib) or lower VT parts of the heart. So your probably seeing a general cardiologist at the moment and I always had both so don't be concerned in asking about a specialist. And as a Heart Transplant recipient there is one more area of cardiologist who specialize in that area too but hopefully you won't need them anytime soon. I had a flu virus that caused my heart to enlarge which started my issues but had a normal life for over 10 years with treatments. And you may just be experiencing something that most people will experience occasionally Arrhythmias. They are actual normal under things like stress, anxiety and many times go away by themselves. Please let me know if I can answer any other questions? Please let me know what you find out? Talked to you soon