Relationships. Not necessarily, issues in all our lives can make us back away from intimacy. If it is chronic however it probably needs to be addressed with the help of a mental health professional. Best.
Not interested? Without more info, it's impossible to say. Are you concerned about your disinterest in romantic relationships? People's timetables for these things are very individual. You may be in a time of life where you're simply more interested in other things. Or you could be dealing with a loss of some kind, and avoiding further involvements. Many more possibilities too . . .
RelationshipDisorder. One question to ask is "are you bothered by it"? If you say you are "not concerned", they you probably don't have a disorder. Usually the criteria for a disorder is that you are bothered by it or it causes significant difficulty in your functioning in life. There are a couple personality disorders where this condition may occur. You could talk with a psychologist to see if there is something.