This is from the london weather station...

People with rheumatoid arthritis, osteoarthritis, fibromyalgia, gout and neuralgia, and those with missing limbs or scars, can experience more pain in stormy weather.

Because tendons, muscles and bones are of differing densities, cold or humid air may cause them to expand or contract in different ways. If there is already swelling, stiffness, inflammation or abnormal mechanics in the joint, as the atmospheric pressure changes so the unequal expansion and contraction of tissues may cause increased pain.

The density of scar tissue is also different from that of normal skin.
It could also be that during colder weather, arthritis sufferers, for example, are less inclined to get outdoors and perform normal activities to keep their joints supple. Depression brought on by dreary weather may also make pain feel worse.