I have been having pain in my arm similar to tendinitis. I notice the more I use it, the less pain I have. But when I walk, I usually do about 2 miles a day, my arm stiffens up and it is really painful to bend my elbow. After I am done walking, and do some range of motion, it works itself out again. It seems to feel better when I do some lifting.
Welcome to Chat Doctor, and Thanks for asking From the medical history you provided it seems that most probably you have sprained muscle in your arm, and it should heal within 3-7 day, however severe strained muscle can take up to 10 weeks to get normal completely and need medical intervention u can Use over-the-counter anti-inflammatory medication, such as ibuprofen. And muscle relaxant like thiocholchicoside after discussing your Chat Doctor.  Slowly increase your level of activity if your pain does not subside in 7-10 days, you feel any numbness there in your arm, if you are unable to move it completely than I would strongly suggest you to see an orthopedic surgeon, to rule out any underlying medical conditions hope it helps Thanks & Get well Soon