Welcome to Android Central! It could very well be the task-killer app. Task-killers are not needed on modern Android devices, and often cause more problems than they claim to solve. Shady companies tend to push these kinds of apps as well, so that they can also include adware and sometimes worse stuff. Android is designed to manage RAM intelligently, so the best thing to do is let it do what it's designed to do. Uninstall that task-killer now.