Why my game's FPS drop every time I change scene (Android Game) ?

If you're "not keeping anything with DontDestroyOnLoad()" surely everything will then get destroyed - hence everything reset when a scene is re/loaded. If on the other hand that (not) was a typo, then when you reload the screen you will have the undestroyed item and the newly initialised item (from loading the scene again) - but unless you have held on to the original object handle/(s), the data will seem to have reset... you would in fact be looking at the new version of the item. The duplication of objects could then be slowing down the game? Just a guess

Are you loading the scenes as additive?

How is it behaving inside the editor? Is it using more memory after you change the scene back? If it does then you might have some references of Objects which can't be garbage collected. This is called a memory leak. This is hard to track down especially when you're not used to manual memory management.