Knee swollen and bruised out of no where ,bruise slowly getting bigger, don't recall no injury , what can it be have not seen doc yet , any ideas?

Unclear: It can be a number of things given your description from a simple contusion or bruise, prepatellar bursitis, cellulitis or something involving the knee joint. You may have sustained unrealized trauma especially if you are active. If you have pain, increased swelling, decreased range of motion and it affects your activity level you should be evaluated.