My volunteer work at the local MS Centre brings me into contact with many people with whom I have to be aware of changes with their symptoms and how they feel. Most have MS, but there are also those with cancer, some with leg ulcers, some having had surgery for broken bones. Most have said something similar to you and are putting it down to the weather, it seems to drain people of any energy they might have had. Obviously this is just anecdotal, but it makes you think.