Could the frozen thawed may have not been compleatly warm or thawed enough? Possibly try making sure that especially in the middle it's compleaty thawed and warm enough about 100.f. I beleive is what a normal rats temp is. If you have a digital heat gun check the temp like that or poke into the belly with your finger. With like a latex glove on or something. Should feel a little warm to you but not hot.