This is what I use along with the task manager that comes with the SGP5. It sounds to me that maybe a updated app may be the cause. Some apps update on their own. Make sure in Google Play that you don't have this option on. The other option is a app corrupted or gone rogue. Sometimes you can figure it out and sometimes you can't.
You can always do a factory reset and start over. Sometimes I find this is best when there are too many apps to figure out.
My phone has been acting this way and then out of blue it stopped. It's a mystery because I changed nothing.