Maybe your Volume Down button is stuck???
To rule out software completely, make a backup. Then reset your phone with one of these tools.
How can I recover/reset/restore my phone software? - Microsoft - USA
Do NOT restore your backup. Run the phone and see if the problem occurs again. Yes, the problem is there? Then it is hardware 99.99% certain.
No problem? Do a Factory Reset... Settings > about > reset your phone > go ahead and restore your backup this time. If the problem pops up it is due to an App or a Setting and you have to sort that out separately. Try to remember the last things you did or added before the problem started...