Weird auto-volume issue

About every other night, I play audio off my phones speakerphone while laying in bed. Since last night, I noticed that the volume was noticeably increased without it being touched! If I were to guess, I'd say it does it once or twice every 2 to 10 minutes.
Also similar issue on Bluetooth - except the volume turns down completely to zero! Kind of getting annoying.. The phone has never been dropped and the case has remained on it for several months. Please help
Thanks

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