Hi, since it did not happen with sport or accident, it is not likely to be a meniscal tear but more likely to be misplaced meniscus, try to relocate it by sitting with your knee at 90 degrees and turn your foot outso it twists at the knee. you might need to get someone to help you if it feel like there is something in the way, you will get a click and then should feel better with straightening it. still take anti inflammatories for a few days. if this fixes it then to avoid it happening again, you need to strengthen your quads.