How do i know if i have cellulitis on my knee? and does it cause bruising of the leg?

Spreading red warmth: This is typically associated with an enlarging area of redness, swelling and warmth. Occasionally one may see involvement of the lymph vessels with red streaks going up to the groin and called lymphangitis. As seen in the picture, some may circle the borders of the redness to monitor the spread into the surrounding normal tissues.

Red, warm, tender: If its red, warm, tender to touch, likely cellulitis. Sometimes deeper tissue injury/infection can cause a bruise appearance also, and sometimes a bruise/hematoma can become infected and cause a cellulitis-- so they can coexist. Best to let your doc make distinction, but if you don't recall any trauma, less likely a bruise.