Cellulitis, a bacterial infection of the skin, causes red, tender skin that is usually swollen and warm. Redness, swelling, pain, itching, bumps are common symptoms of insect bites. An insect sting may be followed by redness, swelling, and itching. A pulmonary embolism, a blood clot lodged in the lung, causes shortness of breath, chest pain, and more. Pulmonary hypertension can cause shortness of breath, a fast heart rate, or lightheadedness.