Although a racing heart during exercise has several different possible causes, pain in one side is often caused by diaphragm cramping, also known as a side stitch. When you first feel the cramp, take a big breath as if you're sucking through a straw, then exhale forcefully like you're blowing up a balloon. Often, side stitches and a racing heart are both signs that you're working yourself too hard during exercise.
Overexertion
If you're new to intense physical activity or have been pushing your limits lately, you may just be working a little too hard. Exercise forces your heart to work harder, which increases your heart rate. This is a good thing, because it improves your heart's condition, but people who aren't in good shape can find it overwhelming at first. Over time, as you become more fit, it won't be as difficult on your heart to do the same level of exercise. In the meantime, slow down just a little and work out at a heart rate you can maintain comfortably.
Left Side Pain
A pain in your left side can indicate something serious -- if the pain is in your upper chest, it could even be a symptom of a heart attack. However, lower left-side pain likely indicates a problem in your abdominal area. For example, a colon condition called diverticulitis is marked by recurring pain in the lower left side, as well as fever. If the pain is reoccurring and doesn't feel like a side stitch, see your doctor for the most accurate diagnosis.
Low Blood Pressure
If you often feel your heart racing during exercise, low blood pressure may be the culprit. When your blood pressure is abnormally low, your heart has trouble getting enough oxygen into your blood. This can lead to an elevated heart rate, as well as dizziness and weakness. In severe cases, low blood pressure can lead to heart and brain damage if left untreated. Medications and dietary changes -- especially increasing your water intake -- can help regulate your blood pressure.
Heart Palpitations
Heart palpitations occur in your chest, throat or neck, and they may feel irregular in addition to a racing or pounding sensation. Palpitations can be a symptom of arrhythmia, or abnormal heart rhythm. A resting heart rate of more than 100 beats per minute signifies a condition called tachycardia. However, your heart rate will likely be higher than that during exercise. Stress-relief techniques -- including low-impact exercise methods such as yoga and tai chi -- can help reduce the incidence of heart palpitations.
