These three combined, indicate a mechanical problem such as a torn cartilage
possibly combined with a fluid collection, known as an effusion, in the knee
You mentioned no trauma, but didn't say whether you are active or not. You didn't mention redness, or increased warmth, so i will assume they are not present.
My first choice is probably what is known as a degenerative torn meniscus. These occur not from trauma, but rather from wear and tear over time.