Extracted wisdom tooth was healing fine, and now it's infected!! HELP please!?

He shouldn't have to cut you open again, he'll probably have a look and make sure that all of the tooth came out. It's likely that you got some food trapped and the infection has flared up. He might flush out the pus and prescribe some antibiotics to fight the infection. You may have to avoid eating on that side so that you don't aggravate the infection more.