Be careful not to get any food in the open hole, but if you start to experience swelling in the area of the extraction, then chances are that you have an infection. You should have gotten some penicillin from the dentist.
Infections in your gums are generally accompanied by a strong pain. If you begin to experience pain that is clearly not just a simple toothache and does not go away, see your dentist. You might also notice unusual redness or whiteness in the gums.