That happened to me intermittently (approx every six months) for over a decade. I'm not so sure my knee cap was at right angles to my leg, although I became rather adept at clicking the knee back into place -- relocating the kneecap?
I'm pretty sure that I did mine in 2000/2001, and it was only diagnosed last year. This is despite going through an X-ray, MRI and exploratory keyhole surgery.