When I walk from a dark room to a well-lit one, and my eyes sting for a moment, what’s causing that sensation of pain?
When you’re in a dark room, the pupils of your eyes are dilated as open as they can get to catch any stray photons so that you can see. When you’re in a bright room, the pupils contract much smaller, so that your eyes don’t take in too much light, which can overwhelm your visual equipment, preventing you from seeing at all (think about when you’re exposed to a very bright light, such as looking straight at the sun, or a brigh camera flash). For the split second after you walk into a bright room from a dark one, your pupils haven’t yet adapted to the light change, and it hurts.
I understand that it hurts, I’ve done it enough times. 
What I wanted to know was why? There doesn’t seem (to me at least) any physical reason why you should feel pain from the sudden change in light. It’s not like photons have mass and are painfully slamming into your eyeballs, right? So what part of the process is making your brain go “ow” in addition to the temporary blindness (to whatever degree)?
The light irritates the sensitive tissues of your retina. Since too much light can cause blindness from damage to the retina, I don’t see why lower levels wouldn’t cause pain. It’s like heat on your skin, methinks: Warmth doesn’t hurt at all. Too much warmth causes a burning sensation. WAY too much warmth hurts like hell and causes serious skin damage.