Sewing Machine "bird nesting" Problems?

I have a Singer Simple sewing machine. I have been using it for years now. I am having a problem with my stitches making a “nest” on the underside. I have completely rethreaded the machine. I have adjusted the tension. I have changed the needle. And I have thoroughly cleaned my machine. I am not sure what else to do. I don’t know what is causing it. If anyone can suggest something else to do it will be greatly appreciated. Thanks in advance!!!