I suggest that you get in to see your primary care doctor
to help you figure out what this could be. I am not sure exactly what is going on based on the symptoms you describe.
First of all, have you been recently exercising vigorously or otherwise engaged in significant physical activity? If so, then probably this is just over exertion and it should get better with some rest and backing off of your training schedule a bit.
If the pain is centered primarily around the knees, then in someone of your age the most likely possibility would be the start of osteoarthritis
. Osteoarthritis is the 'wear and tear' form of arthritis that is common as we age. It often starts with intermittent pain and swelling and stiffness in the major joints (such as the hips and knees) that is worse late in the day (after being active all day). Treatment generally involves initially limiting stress on the joints (while still staying active) and anti inflammatory medications such as ibuprofen.
Start by setting up an appointment with your primary care doctor to discuss this issue. If the pain is severe, or if there are other concerning symptoms, such as noticeable joint swelling, rash, or fevers, you should make the appointment quickly.