Good Afternoon and welcome! I am Dr. Loretta, a licensed veterinarian and I am happy to answer your question.
I am so sorry that it has taken so log to answer your question but I would like to offer some ideas.
Because of your mares age, there are a number of things that could be causing these symptoms and I do believe that there are a couple things going on. It is not unusual for older mares to have digestive distrubances and sometimes it can be attributed to benign internal tumors that move around and when they are in a certain position, will cause mild colic signs. If this is the case, there is nothing you can do except possibly give a dose of Butazolidin and feed a bran mash for a few days.
Your mare probably does have arthritis and weather changes will cause this to appear worse. Start your mare on Glucosamine/Chondritin, available at the farm supply, which will add lubrication to her joints and ease her distress. It will help to give her Butazolidin also.
The skin issue could be many things such as a fungal or bacterial infection, parasite or metabolic. A hypohyroid condition, common in older horses can cause this so have your vet, when you can get ahold of her, to run blood work including a thyroid panel to eb sure. In the meantime, apply a topical ointment such as Betadine daily that will dry out the skin so bacteria and /or fungus cannot grow.
Does that make sense to you?
Please let me know if you have ANY other questions. My goal is to give you 100% satisfaction and if you are not yet satisfied, please REPLY so I can clarify for you.
Sincerely, ***** *****