I Have Had An Unexplained Low Grade Fever That Is

I have had an unexplained low grade fever that is intermittent for the last several months. When it occurs, it is usually in the evening. I have also noticed joint pain in my elbows and knees. Before it used to be just my lower back. Like my tailbone/SI joint area. It would start to hurt out of no where and usually last a week or two and then would go away for several weeks. Sometime months at a time. Usually the joint pain is worse at night and gets better as I get going in the morning. I have also had 3 episodes where I have this mottling rash on my legs. I just associated the rash with the cold. What do you think this could be?