According to an article on WebMD: Stress can increase your perception of pain. "Stress can leave you feeling less able to handle your pain and more easily overwhelmed by it," says Sharon Kolasinski, MD, a rheumatologist and professor of clinical medicine at the University of Pennsylvania. And, having a chronic illness, such as arthritis, can add to your stress level. "Chronic pain is a stressor in itself," says Kolasinski. "But it can be even more stressful when the pain limits your mobility or activities."

The Arthritis Foundation has some great information related to stress and arthritis:

How stress contributes to arthritis

Your body’s stress response triggers the release of chemicals that ready you to face the challenge at hand. Your breathing quickens, your heart rate increases, and your muscles tense in preparation. This reaction is fine in the short term, but when it fires repeatedly, the increased tension in your muscles can amplify your arthritis pain. Stress also sets off the immune system’s inflammatory response. Inflammation is what fuels joint damage in rheumatoid arthritis (RA), lupus, psoriatic arthritis (PsA), ankylosing spondylitis (AS), and other inflammatory forms of the disease. The longer you’re exposed to stress, the more destructive the inflammation can become. In a PLoS One study, people with RA identified stress as a trigger for disease flare-ups.

How arthritis worsens stress

Arthritis symptoms contribute to stress, especially when they’re unrelenting. Constant pain, fatigue, and poor sleep create a vicious cycle. Each symptom worsens the others and adds to the stress you already feel. When you’re tired, you don’t want to exercise. A lack of movement inflames your pain, which makes it harder to sleep. Worrying about future disability, getting pregnant or difficulty paying your medical bills only adds to your stress.

Taken from Arthritis Foundation