<s>[INST] When cycling after about 20 mins or so I feel a fluttering feeling in my upper area just below my left side under my collar bone rib area. It feels like I need to take a big breath of air and adrenaline races around my body. I feel like I need to stop and get myself together. I m 32, and with possible ( not diagnosed yet) costochondritis. It makes me nervous to excersise to my full potential an I m afraid it might be my heart. [/INST] Thanks for your question on Chat Doctor. I can understand your situation and problem. Since you are having left sided chest discomfort, we should first rule out cardiac cause. And fluttering feeling can be due to tachycardia and arrhythmia. So better to consult cardiologist and get done1. ECG2. 2d echo3. Stress test. If all of the above are normal then no need to worry much for cardiac cause. You may have anxiety mostly. So avoid stress and tension. Be relax and calm. Don't worry, you can continue your exercise. </s>