doctor could be a good idea, but a physiotherapist or a sports therapist could be worth thinking about as they specialise in soft tissue injuries - and rather than just giving you painkillers and anti-inflammatories, as a suspect a doctor probably would, they will test the joint and try to work out the cause of the problem (rather than just treat the symptoms). It could be any number of things, probably just something simple like tightened muscles around the joint that a stretching program will sort out quite quickly. I had the same problem when i was cycling a lot, hamstrings shortened and knees when a bit odd (and sore)- but was easily fixed (by a sports therapist)!