Why does my phone get little to no reception with AT&T?

I have an iphone 3Gs, and have AT&T. I have had the same plan since I got my phone, and have never had this problem. I would get perfect service in every room of my house, even my basement! However, now the maximum amount of bars I get in my home is 2, I constantly have dropped calls (Which hasn't happened before) and only get OK reception when I am at a public place. (Chilis, Home Depot, Target)(3/4 bars)I have checked my parents cell phones as well, and their reception sucks as well. Does anybody know what is wrong with AT&T?