The speaker is getting high, and instead of simply enjoying it, a worry creeps in: that this pleasure has a price. The phrase "debtor to joy" turns happiness into something borrowed. Joy isn't a gift here but a loan, and the speaker feels they may have to pay it back somehow. It's a small, honest moment of guilt or unease sitting right beside delight.
The second stanza shifts the image. The speaker compares the fear, or perhaps the whole thought, to a question "discharged" into the quiet, the way you might fire a shot or release something you can't take back. Nobody answers. The silence just absorbs it. That leaves the mood hushed, a little lonely, and thoughtful rather than dramatic. The poem doesn't tell us whether the debt is real. It stays in that uncertain pause, where pleasure and doubt share the same still room, and lets the reader sit there too.