I excercised for first time last week. Did nothing way too strenuous. Last 5 days or so though the back of my upper arm above my elbow has been tender and into my armpit area,burny too. So not sure if its from pulled muscle or my body fighting infection. My dad always said about his armpits bein tender and that it was from his body fighting off a cold or something. My right armpit i think was tender before,but not anymore. Not sure if related but i have had a weird sinus thing going on last few days. Not full blown cold but almost..throat sore from sinus congestion,etc..