Lack of exercise can lead to joint pain as much and as often as injury during exercise. Exercise does a few things, one is to strengthen the surrounding muscle and ligaments which support the skeletal structure. Joints need the support of the attending muscles and tendons/ligaments or they will easily slip and injure - even sleeping can lead to injury of joints if your too flabby. Being over weight is only one kind of symptom of being out of shape - and contrary to popular belief many people who are over weight are fit and in shape and also have a lot of muscle which supports their skeletal structure. So that is not always a sign of unfitness of body. Plenty of people do injure themselves and do not know it at the time - often one can just turn the right way and do damage and the onset of pain can be hours or a day away. And there are a number of diseases that can lead to joint pain as well - some may not cause the pain all the time, may "flare up" at different times with periods of no discomfort between. This requires a doctors attention, needs tests, pictures (X-ray, possibly MRI) to see what is going on inside the joint. Bone spurs come to mind, so too arthritis. Arthritis is not an "old person's disease" many teenagers and young adults have it. Serious pain should always be seen by a doctor - if not the first time then the second time it takes place. Three is not always the charm in health issues like this. There are times when waiting too long can have severe long term consequences.