I'm old and get hurt often which leads me to a quick(ish) question: even though you didn't have an impact or specific point when an injury can be diagnosed, have you been doing more physical things lately? started working out, running, skiing(if you have snow) more recently?
like a previous poster mentioned, this could be associated with a ligament issue. that being said, it might not be the end of the world. Sometimes when you start something new physically i.e. the start of a sports season, you stress your joints/muscles/body differently (obvious statement, i know) what this could lead to is inflamed ligaments/joints that were just pushed a liiiitle to hard the first time. the only thing (if this is the case) that you can do is chill out a little and then ease your way back into whatever activity you were doing (preferably with more stretching before and after and maybe a little more conditioning)
if you notice a change in the type of pain you should def go back to your doc asap and if it persists for more than a week or two you also should go back