I Had A Tooth Extraction About 3-4 Weeks Ago, When

I had a tooth extraction about 3-4 weeks ago, when they pulled it they said their was a small hole from the extraction to my sinus'. I now have what appears to look like a small thin layer filled with blood where my tooth was. It looks like a bloos blister and if i barely push on it i have a lit of pressure to my nose and head. What could be doing on, is it dangerous? Should i go to the ER?