If you didn't bash into some gym equipment, have you twisted your knee? How's the ITB? Is there a remote chance you had some sort of minor, or seemed minor, knee injury that's causing the bruising?

I find that if I pinch something I'm more likely to bruise than if I bash into it straight on.

How's your blood sugar? I've been checked multiple times for diabetes because of my propensity to bruise easily, and heal slowly.