Work injury after almost 3 months

I was injured at work back in August and still find myself in a lot of pain. About 3 to 4 hours into my Monday shift my should starts to hurt really badly. It hurts to the point I call in sick because I just can't handle the pain. Even with a sit/stand/walk order that allows me to get up and move around as needed, I just can't seem to make any headway. I noticed they other day that I couldn't even carry a large bottle of water with that hand because the shoulder becomes strained. I am terrified I will lost my job based on the days I must take for the pain from the work injury.