What can be causing pain in my knee for over a week? (no known injury)

[senjougahara]

Hello. Last week I felt a pain in my right knee when walking downstairs. That happens sometimes and usually goes away the next day but this time it didn't and has been going on for over a week now. The pain is dull and not very strong. I noticed it happens when I shift more weight to the right leg- mostly when walking up and down stairs or when  squatting and then standing up. It does not hurt when: I stand (even for a long time) or walk. It only hurts when bent and the pain gets worse if I am sitting with my legs bent for a long time. I can also hear/feel a quiet click sound when bending the knee. There has been no swelling, redness, heat and other signs of inflammation. At my job I stand for 8 hours and walk and after working two days the pain and discomfort were lessened. However, yesterday at work the knee felt numb rather than painful and my calf felt tense with a mild pulling sensation but today that is gone and it is the usual mild pain from before.
It sounds like a 'wear and tear'' issue but I am only 24 and I don't ( and never did) jog or play sports that would cause strain on my joints (just occasionally going to the gym, swimming, some yoga and dancing to keep healthy), I do not have close relatives with joint or cartilage issues like arthritis, I do not recall injuring the knee but it's possible I hit it maybe without noticing. I do however work a standing and walking job on and off for nearly 10 years now (summer jobs, internships etc), I had ''growing pains'' in my knees at night as a child that never really stopped (now they are weather based) and I had to cycle often since I moved to The Netherlands but only short 10 minute trips and I stopped cycling about a year ago.
What can this be? Is it possible I just hit it in some way and it will go away? Should I go to a doctor or wait a bit more to see if it gets better (or worse)?

Thank you.