6S' earpiece very low when talking on the phone

This starting happening around 3-4 days ago. My phone has full volume and everything, the speaker works fine, but whenever I receive a call or make one, I can barely hear anything through the earpiece. They can hear me, but I can barely hear them unless I put the call on speaker or plug in my earphones.

I did the usual troubleshooting: turning phone off/on, checking volume levels, etc. but nothing. Any ideas why this is happening?