You remembered to put the foot down before stitching? You rethreaded with foot up? Your bobbin unwinding right direction? It really sounds like your thread jumped out of the take up arm. Feel around the bobbin race for any nicks--also the needle plate opening...thread can hang up there and cause a mess. Nicks can be filed down with emery cloth. Usually when the nest is on the bottom, it means the problem is on top...in the threading or the tensions.