I woke up with a swollen left eye that is slightly red and kind of tender to the touch. If I put a small amount of pressure on it my whole eye socket hurts. Do you have any idea what might be happening?

Sometimes when you fall into a deep sleep and your in a position that irritates you like sleeping on your hand and a knuckle contacts your eye you can wake up that way as your eye and surrounding area try to fight it off. Another thing is if you have had irritation and have been rubbing your eyes a lot the same thing can happen. If it gets worse or you start to lose vision then see a doctor right away.

Maybe it's conjunctivitis ("pink eye")?

Pinkeye (Conjunctivitis) - WebMD | Causes, Symptoms, Diagnosis, and Treatment (http://www.webmd.com/eye-health/tc/pinkeye-topic-overview)