You are small. Your weight may be part of the problem. How many calories do you take in, in a day? How much protein do you get a day? How much water do you drink? I would try to work on eating more obviously not junk food but more food in general and drinking more water and see if that helps. If your blood pressure runs on the low side you may want to consider adding a bit more sodium to your diet but only do so if your bp is consistently low. You can also ask the doctor for a small dose of beta blocker and see if it helps your heart calm down a bit. Other than that the heart becomes conditioned over time. So start out slow with just walking and see if you can slowly build up to a fast walk, then a slow jog. Make sure you have eaten a half hour prior and have drank some water. Bring water with you. I do understand that IST is a bit more tricky to treat but try a few of the things I suggested and see if it helps. Best of luck and feel better soon. Take care.
One other thing. If you haven't had your thyroid checked you may want to get it evaluated. If you have hyperthyroidism that could be part of the problem.