Unexpected battery drainage

My iPhone 7 Plus goes through this cycle when it’s battery starts to drain very fast for a few days and then it becomes normal for the next few days. I mean, during those days it’s battery starts to drain fully in as less as 6 hours even when I’m not using it. When it returns to normal, the battery lasts for almost 1.5 days which is its normal performance. I don’t understand what type of phenomenon is this, because my iPhone usage does not change at all during this period. This has really started to annoy me because during that fast drainage period I have to always keep my phone on charging and can’t use it as well as I need to. Has anyone come across this issue and explain to me what causes it and how to solve it for good ?