I suffer from 2 things which have caused symptoms similar to yours: a digestive disease called Celiac Disease, and a problem with heart rate called Orthostatic Intolerance, aka "POTS". My heart rate is steady but very fast, especially when standing or walking. It can be as high as in the 150s just because I've stood up. The way to diagnose the POTS is through having a tilt table test, a non-invasive test that monitors your blood pressure and heart rate while you lie down and stand up for a period of time. First, though, you should see a doctor to rule out any structural abnormalities with your heart, by having an EKG and echocardiogram, and chest X-ray. The Celiac Disease can be found in the blood, then confirmed through having a small intestine biopsy. Either of these match your symptoms, but then again, so do a hundred or more other illnesses...