What knee injury do I have if instability?

Usually a ligament. Ligament damage is the usual cause of instability-acl, mcl, etc. See an orthopedist.
Quad weakness. In addition to the above, more often than not a knee instability is merely a "giving way" sensation that feels like your legs are falling underneath. It gives the sensation of falling forward and of an unstable knee. This is quad muscle weakness and inhibition, where your body is unable to tighten the muscle in time to hold you up. Quad exercises or therapy can improve this.