Knee pain is actually a very common complaint. While many causes are not very troublesome or serious, there are some that require attention in order to prevent long term problems. I would recommend that you see your primary care doctor
at once to address this.
Any joint pain
requires evaluation. Keep in mind that when a doctor evaluates a joint, he or she must make sure the pain is not referred from the joint above (the hip) or below (the ankle). As you've had hip problems in the past, this is something to consider.
When thinking of joint problems, one must consider if its a single joint or multiple joints. Multiple joints (like hip and knee pain) can because by autoimmune disease like rheumatoid arthritis
. This should be evaluated by your doctor.
It does sound like your pain is more from the joint itself. This can be from misalignment or instability of the joint. What you describe could be consistent with patellofemoral syndrome, where the kneecap (patella) is misaligned. Physical therapy can be helpful for this. Other instability problems can occur from ligament, muscle or meniscal (cartilage) tears.
I would recommend that you talk to your primary care doctor. This should be evaluated.