Hi, What Do You Think Happened To My Arm, I've

hi, what do you think happened to my arm, i've been suffering with pain on my upper arm mid way up from the elbow and down from the shoulder. I do not remember hitting nor pulled any muscle, but it seems more like a tear. I cannot move my arm back any type of movement backwards or even trying to stretch it back or even something heavy i have to let it go. i just noticed today," dont mean to be a bit graphic" so i apologize.. i used the bathroom and just pulling my pants down , i suddenly grabbed hold of my arm and i was in so much pain it took a bit more longer for the pain to go away. What can you tell me about this pain. I am going to get it x-ray done, i just want to know so i know what to expect just to ease some questions right now. Please help me identify what the issue might be.