I'd install 3C Toolbox from the Play Store. Use its Tasks Manager tab to check what app(s) might be using up all available memory and CPU. The button in the lower left corner of this tab lets you choose the criteria for sorting the list, e.g. Used Memory, CPU Time, etc.
Be careful with the rest of the functions of this app. Many are not obvious and somewhat technical. There are no confirmation prompts, especially with the One-Click features. However, root privileges is required for the potentially dangerous functions. So, on a non-rooted device, it's likely more of an inconvenience if this app is used incorrectly, or some data loss if you erase/wipe/delete something unintentionally.