Seidio Extended Battery - Phone randomly shuts off

[nonstop]

My phone is a year old and the battery stopped holding a charge. The battery was never great to begin with, and now even very casual use will kill it in half a day. So I decided to order the Seidio extended battery, it arrived last night and I charged it over night. Now my phone keeps randomly shutting off. Usually when I flip open the keyboard. The battery appears to be seated well. Does anyone have a clue what may be causing this? Faulty battery? Thanks!