The IT band runs from the hip, down the side of the thigh and inserts into the side of the knee. It is possibly the source of your woes but the way you describe it it sounds more likely to be something to do with either the kneecap or the tendon which joins the kneecap to the lower leg or both. People will get osteoarthritis as they age, it is an unfortunate part of the ageing process and not related to rheumatoud arthritis. One of the most common places to get it is behind the knee cap because it is a joint that is used very often everyday with our body weight on top of it. The best way to keep it in check is frequent, regular, not over-the-top exercise (over-the-top being things like running on tarmac/pavement for hours on end).
However, with all that being said, it is unusual that your pain has come on without any obvious aggravating activity. If it persists then get thee to a physician. Many hospital trusts let you self refer to a physio if that’s your preference.
Best cure for IT band = foam roller, lying on your side, rolling it up and down the length of the outside of your thigh from your hip to your knee with it between your thigh and the floor. It if hurts like absolute bloody murder then you’re doing it right!!