Help! I woke up with a warm phone and 0% battery from a 77% charge :(

[akoctophe]

I have a habit of putting my S3 Mini under my pillow upon sleeping. I use juice defender ultimate for the past 2 months and it really saves my phone some charge. But this time, when I checked my phone this morning, I found out that it is kinda warm and upon pushing the power/screen off button, I found out that it was 0% from a 77% of charge. Anyone knows what to point out on this problem? It also happened the other day, I woke up to a warm phone with a 6% charge from a 55%. Other than that, it never happened again until this morning. Anyone knows the problem? Note: I have a habit of closing all background apps (and I also know that some apps would automatically open up again) before sleeping. So please help me