Which tooth is it? If its an upper back tooth (molar), then its possible that during the extraction, you may have had a small sinus
perforationEsophageal perforation
Gastrointestinal perforation. Its not an unsual occurence for a sinus perforation to develop.
I would recommend going back to your dentist for an evaluation. If it is a sinus perforation, he will prescribe antibiotics to prevent a sinus infection, perhaps a nasal decongestant so you don't blow your nose and may go back into the area or suture the socket up to help with the healing.
If the sinus perforation is large, he may send you to an oral surgeon to have a flap placed in which they slide the gums over to cover the socket.