Why does my shoulder hurt when I lift my arm?

The shoulder is a trade: it gives you the most movement of any joint in exchange for stability, and the tendons of the rotator cuff run through a tight arch of bone at the top. When those tendons or the cushioning sac (bursa) get irritated - by repetitive overhead work, a gym increase, or just years of use - they swell, and lifting your arm pinches the swollen tissue against the arch. The NHS's symptom guide matches your pattern exactly: pain that is often worse while using your arm or shoulder points to tendonitis, bursitis, or impingement. Many people feel it most in a painful arc between shoulder height and overhead, or lying on that side at night. The good news from both the NHS and MedlinePlus: this pattern usually improves with physiotherapy and activity changes, not surgery. The plan: ease off the aggravating movements for a couple of weeks (overhead lifting, heavy bags on that shoulder), keep the arm gently moving so it does not stiffen - a frozen shoulder is the complication of a shoulder you stop using - use anti-inflammatories if they suit you, and start gentle pendulum and stretching exercises, the kind a physio prescribes. The NHS's line: see a GP if the pain is getting worse or has not improved after 2 weeks, or if moving the arm is very difficult - and get urgent help for sudden severe pain after an injury, or an arm you cannot lift at all. Pymander is a free AI doctor, 24/7 by text, that can help you map the pattern.