Is my labret piercing infected? or is this how it normally develops..?

The skin on the inside of your mouth is trying to still heal. It doesn't know what a piercing is so it's trying to heal over the thing. Make sure it doesn't do that, don't let the skin keep trying to cover the back. As for the pain I'm not really sure about that so you should go ask your piercer. Oh and make sure you're using non-alcoholic mouth wash.